This work was supported by the National Natural Science Foundation of China (grant no. 31600250, Y.Z.), Science and Technology Projects of Guangzhou City (grant no. 201804020015, H.N.), and the China Agricultural Research System (grant no. CARS-04-PS09, H.N.).

1. Primer Design
<ol>
	<li>Design gene-specific primers for reverse transcription (RT) using PCR primer design software (Table of Materials) based on the general rules of primer design17. 
	NOTE: RT primers are highly specific to the target transcripts, and they are generally anchored on the 5&#8217; part of coding sequences (mature mRNAs and precursor RNAs), or ~500&#8211;600 nt downstream of the anticipated 5&#8217; end (18S and 26S rRNAs).</li>
	<li>Desi...

<ol>
	<li>Zhang, Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Major+contribution+of+transcription+initiation+to+5&#39;-end+formation+of+mitochondrial+steady-state+transcripts+in+maize.">Major contribution of transcription initiation to 5&#39;-end formation of mitochondrial steady-state transcripts in maize.</a> RNA Biology. 16 (1), 104-117 (2019).</li><li>Choi, B. Y., Acero, M. M., Bonen, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mapping+of+wheat+mitochondrial+mRNA+termini+and+comparison+with+breakpoints+in+DNA+homology+among+plants.">Mapping of wheat mitochondrial mRNA termini and comparison with breakpoints in DNA homology among plants.</a> Plant Molecular Biology. 80 (4-5), 539-552 (2012).</li><li>Calixte, S., Bonen, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Developmentally-specific+transcripts+from+the+ccmFN-rps1+locus+in+wheat+mitochondria.">Developmentally-specific transcripts from the ccmFN-rps1 locus in wheat mitochondria.</a> Molecular Genetics and Genomics. 280 (5), 419-426 (2008).</li><li>Kuhn, K., Weihe, A., Borner, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Multiple+promoters+are+a+common+feature+of+mitochondrial+genes+in+Arabidopsis.">Multiple promoters are a common feature of mitochondrial genes in Arabidopsis.</a> Nucleic Acids Research. 33 (1), 337-346 (2005).</li><li>Jonietz, C., Forner, J., Holzle, A., Thuss, S., Binder, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=RNA+PROCESSING+FACTOR2+is+required+for+5'+end+processing+of+nad9+and+cox3+mRNAs+in+mitochondria+of+Arabidopsis+thaliana.">RNA PROCESSING FACTOR2 is required for 5' end processing of nad9 and cox3 mRNAs in mitochondria of Arabidopsis thaliana.</a> ThePlant Cell. 22 (2), 443-453 (2010).</li><li>Stoll, B., Stoll, K., Steinhilber, J., Jonietz, C., Binder, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mitochondrial+transcript+length+polymorphisms+are+a+widespread+phenomenon+in+Arabidopsis+thaliana.">Mitochondrial transcript length polymorphisms are a widespread phenomenon in Arabidopsis thaliana.</a> Plant Molecular Biology. 81 (3), 221-233 (2013).</li><li>Mulligan, R. M., Lau, G. T., Walbot, V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Numerous+transcription+initiation+sites+exist+for+the+maize+mitochondrial+genes+for+subunit+9+of+the+ATP+synthase+and+subunit+3+of+cytochrome+oxidase.">Numerous transcription initiation sites exist for the maize mitochondrial genes for subunit 9 of the ATP synthase and subunit 3 of cytochrome oxidase.</a> Proceedings of the National Academy of Sciences of the United States of America. 85 (21), 7998-8002 (1988).</li><li>Lupold, D. S., Caoile, A. G., Stern, D. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+maize+mitochondrial+cox2+gene+has+five+promoters+in+two+genomic+regions,+including+a+complex+promoter+consisting+of+seven+overlapping+units.">The maize mitochondrial cox2 gene has five promoters in two genomic regions, including a complex promoter consisting of seven overlapping units.</a> Journal of Biological Chemistry. 274 (6), 3897-3903 (1999).</li><li>Yan, B., Pring, D. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Transcriptional+initiation+sites+in+sorghum+mitochondrial+DNA+indicate+conserved+and+variable+features.">Transcriptional initiation sites in sorghum mitochondrial DNA indicate conserved and variable features.</a> Current Genetics. 32 (4), 287-295 (1997).</li><li>Rapp, W. D., Lupold, D. S., Mack, S., Stern, D. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Architecture+of+the+maize+mitochondrial+atp1+promoter+as+determined+by+linker-scanning+and+point+mutagenesis.">Architecture of the maize mitochondrial atp1 promoter as determined by linker-scanning and point mutagenesis.</a> Molecular and Cellular Biology. 13 (12), 7232-7238 (1993).</li><li>Rapp, W. D., Stern, D. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+conserved+11+nucleotide+sequence+contains+an+essential+promoter+element+of+the+maize+mitochondrial+atp1+gene.">A conserved 11 nucleotide sequence contains an essential promoter element of the maize mitochondrial atp1 gene.</a> The EMBO Journal. 11 (3), 1065-1073 (1992).</li><li>Forner, J., Weber, B., Thuss, S., Wildum, S., Binder, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mapping+of+mitochondrial+mRNA+termini+in+Arabidopsis+thaliana:+t-elements+contribute+to+5'+and+3'+end+formation.">Mapping of mitochondrial mRNA termini in Arabidopsis thaliana: t-elements contribute to 5' and 3' end formation.</a> Nucleic Acids Research. 35 (11), 3676-3692 (2007).</li><li>Binder, S., Stoll, K., Stoll, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Maturation+of+5'+ends+of+plant+mitochondrial+RNAs.">Maturation of 5' ends of plant mitochondrial RNAs.</a> Physiologia Plantarum. 157 (3), 280-288 (2016).</li><li>Hang, R. L., Liu, C. Y., Ahmad, A., Zhang, Y., Lu, F. L., Cao, X. F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Arabidopsis+protein+arginine+methyltransferase+3+is+required+for+ribosome+biogenesis+by+affecting+precursor+ribosomal+RNA+processing.">Arabidopsis protein arginine methyltransferase 3 is required for ribosome biogenesis by affecting precursor ribosomal RNA processing.</a> Proceedings of the National Academy of Sciences of the United States of America. 111 (45), 16190-16195 (2014).</li><li>Wang, H. Q., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Maize+Urb2+protein+is+required+for+kernel+development+and+vegetative+growth+by+affecting+pre-ribosomal+RNA+processing.">Maize Urb2 protein is required for kernel development and vegetative growth by affecting pre-ribosomal RNA processing.</a> New Phytologist. 218 (3), 1233-1246 (2018).</li><li>Maloney, A. P., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Identification+in+maize+mitochondrial+26S+rRNA+of+a+short+5'-end+sequence+possibly+involved+in+transcription+initiation+and+processing.">Identification in maize mitochondrial 26S rRNA of a short 5'-end sequence possibly involved in transcription initiation and processing.</a> Current Genetics. 15 (3), 207-212 (1989).</li><li>Green, R. M., Sambrook, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Molecular Cloning: A Laboratory Manual. , (2012).</li><li>Canino, G., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Arabidopsis+encodes+four+tRNase+Z+enzymes.">Arabidopsis encodes four tRNase Z enzymes.</a> Plant Physiology. 150 (3), 1494-1502 (2009).</li><li>Gobert, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+single+Arabidopsis+organellar+protein+has+RNase+P+activity.">A single Arabidopsis organellar protein has RNase P activity.</a> Nature Structural, Molecular Biology. 17, 740-744 (2010).</li><li>Gutmann, B., Gobert, A., Giege, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=PRORP+proteins+support+RNase+P+activity+in+both+organelles+and+the+nucleus+in+Arabidopsis.">PRORP proteins support RNase P activity in both organelles and the nucleus in Arabidopsis.</a> Genes &amp; Development. 26 (10), 1022-1027 (2012).</li><li>Stern, D. B., Goldschmidt-Clermont, M., Hanson, M. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Chloroplast+RNA+metabolism.">Chloroplast RNA metabolism.</a> Annual Review of Plant Biology. 61, 125-155 (2010).</li></ol>

The authors have nothing to disclose.

In a previous study, total and mitochondrial RNAs from cell suspension culture of Arabidopsis were used to map mitochondrial transcript termini by cRT-PCR, and similar results were obtained12. However, only enriched mitochondrial RNA was used to map mitochondrial transcript termini in many other studies1,2,3,9. We found that the enrichment of mitochondrial RNA i...

In plant mitochondria, many mature and precursor RNAs are accumulated as multiple isoforms, and the steady-state transcripts can be divided into two groups based on the difference at their 5&#39; ends1,2,3,4. The primary transcripts have 5&#39; triphosphate ends, which are derived from transcription initiation. By contrast, the processed transcripts have 5&#39; monophosphate generated by post-transcriptional processing. Discrimination and mapping of the two types of transcripts are important to unravel the molecular mechanisms underlying transcription and transcript end maturation.
To distinguish between the primary and processed transcripts in plant mitochondrion, four major strategies have been developed. The first strategy is to pre-treat the mitochondrial RNAs with tobacco acid pyrophosphatase (TAP), which converts 5&#39; triphosphate to monophosphate and enables primary transcripts to be circularized by RNA ligase. The transcript abundances of TAP-treated and non-treated RNA samples are then compared by rapid amplification of cDNA ends (RACE) or circular RT-PCR (cRT-PCR)2,3,4. In the second strategy, processed transcripts are firstly depleted from mitochondrial RNAs using terminator 5&#39;-phosphate-dependant exonuclease (TEX), and the primary transcripts left are then mapped by primer extension analysis5,6. The third strategy is to pre-cap the primary transcripts using guanylyl transferase, and then the position of triphosphated 5&#39; termini is determined by primer extension together with ribonuclease or S1 nuclease protection analysis7,8,9. Different from those depending on the presence/absence of 5&#39; triphosphate, the fourth strategy combines in vitro transcription, site-directed mutagenesis, and primer extension analysis to characterize the putative promoters and determine the transcription initiation sites8,10,11. By using these strategies, many primary and processed transcripts have been determined in plant mitochondria.
However, several studies have reported that the 5&#39; triphosphate of primary transcripts were unstable, and they were easily converted to monophosphate for unknown reason2,4,12,13. This problem hinders a clear discrimination of the two types of transcripts by using techniques depending on the presence/absence of 5&#39; triphosphate, and previous efforts to systematically discriminate between the primary and processed transcripts in plant mitochondria failed2,12.
In this protocol, we combine cRT-PCR, RNA 5&#39; polyphosphatase treatment, RT-qPCR, and Northern blot to systematically distinguish the primary and processed transcripts stably accumulated in maize (Zea mays) mitochondrion (Figure 1). cRT-PCR allows simultaneous mapping of 5&#39; and 3&#39; extremities of a RNA molecule, and it is usually adapted to map transcript termini in plants2,12,14,15. RNA 5&#39; polyphosphatase could remove two phosphates from the triphosphated 5&#39; termini, which makes the primary transcripts available for self-ligation by RNA ligase. Previous studies showed that mature 26S rRNA in maize had processed 5&#39; terminus, and it was insensitive to RNA 5&#39; polyphosphatase1,16. To minimize the influence of unstable triphosphate at primary 5&#39; termini, the 5&#39; polyphosphatase-treated and non-treated RNAs are normalized by mature 26S rRNA, and the primary and processed transcripts are then differentiated by comparing the cRT-PCR products obtained from the two RNA samples. The cRT-PCR mapping and discrimination results are verified by Northern blot and RT-qPCR, respectively. Finally, alternative primers are used to amplify those transcripts detected in Northern blot but not by cRT-PCR. By using this cRT-PCR-based strategy, most steady-state transcripts in maize mitochondrion have been differentiated and mapped1.

<table>
	<tbody>
		<tr>
			<td>Acetic acid</td>
			<td>Aladdin, China</td>
			<td>A112880</td>
			<td>To prepare 1x TAE buffer</td>
		</tr>
		<tr>
			<td>Applied Biosystems 2720 Thermal Cycler</td>
			<td>Thermo Fisher Scientific, USA</td>
			<td>4359659</td>
			<td>Thermal cycler for PCR amplification</td>
		</tr>
		<tr>
			<td>Ascorbic acid</td>
			<td>Sigma-aldrich, USA</td>
			<td>V900134</td>
			<td>For preparation of extraction buffer</td>
		</tr>
		<tr>
			<td>Biowest Agarose</td>
			<td>Biowest, Spain</td>
			<td>9012-36-6</td>
			<td>To resolve PCR products and RNAs</td>
		</tr>
		<tr>
			<td>Bovine serum albumin</td>
			<td>Sigma-aldrich, USA</td>
			<td>A1933</td>
			<td>For preparation of extraction buffer</td>
		</tr>
		<tr>
			<td>Bromophenol blue</td>
			<td>Sigma-aldrich, USA</td>
			<td>B8026</td>
			<td>For preparation of loading buffer for agarose gel electrophoresis and Northern blot</td>
		</tr>
		<tr>
			<td>DEPC</td>
			<td>Sigma-aldrich, USA</td>
			<td>V900882</td>
			<td>Deactivation of RNase</td>
		</tr>
		<tr>
			<td>DIG Northern starter kit</td>
			<td>Roche, USA</td>
			<td>12039672910</td>
			<td>For DIG-RNA labeling and Northern blot. This kit contains the reagents for transcription-labeling of RNA with DIG and T7 RNA polymerase, hybridization and chemiluminescent detection.</td>
		</tr>
		<tr>
			<td>EDTA</td>
			<td>Sigma-aldrich, USA</td>
			<td>V900106</td>
			<td>For preparation of extraction buffer and 1x TAE buffer</td>
		</tr>
		<tr>
			<td>EGTA</td>
			<td>Sigma-aldrich, USA</td>
			<td>E3889</td>
			<td>For preparation of wash buffer</td>
		</tr>
		<tr>
			<td>Gel documentation system</td>
			<td>Bio-Rad, USA</td>
			<td>Gel Doc XR+</td>
			<td>To image the agarose gel</td>
		</tr>
		<tr>
			<td>Glycerol</td>
			<td>Sigma-aldrich, USA</td>
			<td>G5516</td>
			<td>For preparation of loading buffer for agarose gel electrophoresis</td>
		</tr>
		<tr>
			<td>GoldView II (5000x)</td>
			<td>Solarbio,. China</td>
			<td>G8142</td>
			<td>DNA staining</td>
		</tr>
		<tr>
			<td>Hybond-N+, Nylon membrane</td>
			<td>Amersham Biosciences, USA</td>
			<td>RPN119</td>
			<td>For Northern blot</td>
		</tr>
		<tr>
			<td>Image Lab</td>
			<td>Bio-Rad, USA</td>
			<td>Image Lab 3.0</td>
			<td>Image gel, and compare the abundance of PCR products.</td>
		</tr>
		<tr>
			<td>KH2PO4</td>
			<td>Sigma-aldrich, USA</td>
			<td>V900041</td>
			<td>For preparation of extraction buffer</td>
		</tr>
		<tr>
			<td>KOH</td>
			<td>Aladdin, China</td>
			<td>P112284</td>
			<td>For preparation of extraction buffer</td>
		</tr>
		<tr>
			<td>L-cysteine</td>
			<td>Sigma-aldrich, USA</td>
			<td>V900399</td>
			<td>For preparation of extraction buffer</td>
		</tr>
		<tr>
			<td>Millex</td>
			<td>Millipore, USA</td>
			<td>SLHP033RB</td>
			<td>To sterile extraction and wash buffers by filtration</td>
		</tr>
		<tr>
			<td>Miracloth</td>
			<td>Calbiochem, USA</td>
			<td>475855-1R</td>
			<td>To filter the ground kernel tissues</td>
		</tr>
		<tr>
			<td>MOPS</td>
			<td>Sigma-aldrich, USA</td>
			<td>V900306</td>
			<td>For preparation of running buffer for Northern blot</td>
		</tr>
		<tr>
			<td>NanoDrop</td>
			<td>Thermo Fisher Scientific, USA</td>
			<td>2000C</td>
			<td>For RNA concentration and purity assay</td>
		</tr>
		<tr>
			<td>NaOH</td>
			<td>Sigma-aldrich, USA</td>
			<td>V900797</td>
			<td>For preparation of wash buffer</td>
		</tr>
		<tr>
			<td>pEASY-Blunt simple cloning vector</td>
			<td>TransGen Biotech, China</td>
			<td>CB111</td>
			<td>Cloning of the gel-recovered band. It contains a T7 promoter several bps upstream of the insertion site.</td>
		</tr>
		<tr>
			<td>Phanta max super-fidelity DNA polymerase</td>
			<td>Vazyme, China</td>
			<td>P505</td>
			<td>DNA polymerase for PCR amplification</td>
		</tr>
		<tr>
			<td>Polyvinylpyrrolidone 40</td>
			<td>Sigma-aldrich, USA</td>
			<td>V900008</td>
			<td>For preparation of extraction buffer</td>
		</tr>
		<tr>
			<td>Primer Premier 6.24</td>
			<td>PREMIER Biosoft, USA</td>
			<td>Primer Premier 6.24</td>
			<td>To design primers for reverse transcription and PCR amplification</td>
		</tr>
		<tr>
			<td>PrimeScript II reverse transcriptase</td>
			<td>Takara, Japan</td>
			<td>2690</td>
			<td>To synthesize the first strand cDNA</td>
		</tr>
		<tr>
			<td>PureLink RNA Mini kit</td>
			<td>Thermo Fisher Scientific, USA</td>
			<td>12183025</td>
			<td>For RNA purificaion</td>
		</tr>
		<tr>
			<td>RNA 5&#39; polyphosphatase</td>
			<td>Epicentre, USA</td>
			<td>RP8092H</td>
			<td>To convert 5&#39; triphosphate to monophosphate</td>
		</tr>
		<tr>
			<td>RNase inhibitor</td>
			<td>New England Biolabs, UK</td>
			<td>M0314</td>
			<td>A component of RNA self-ligation and 5&#39; polyphosphatase treatment reactions, and it is used to inhibite the activity of RNase.</td>
		</tr>
		<tr>
			<td>Sodium acetate</td>
			<td>Sigma-aldrich, USA</td>
			<td>V900212</td>
			<td>For preparation of running buffer for Northern blot</td>
		</tr>
		<tr>
			<td>Sodium chloride</td>
			<td>Sigma-aldrich, USA</td>
			<td>V900058</td>
			<td>To prepare 20x SSC</td>
		</tr>
		<tr>
			<td>SsoFas evaGreen supermixes</td>
			<td>Bio-Rad, USA</td>
			<td>1725202</td>
			<td>For RT-qPCR</td>
		</tr>
		<tr>
			<td>T4 RNA Ligase 1</td>
			<td>New England Biolabs, UK</td>
			<td>M0437</td>
			<td>For RNA circularization</td>
		</tr>
		<tr>
			<td>Tetrasodium pyrophosphate</td>
			<td>Sigma-aldrich, USA</td>
			<td>221368</td>
			<td>For preparation of extraction buffer</td>
		</tr>
		<tr>
			<td>TIANgel midi purification kit</td>
			<td>Tiangen Biotech, China</td>
			<td>DP209</td>
			<td>To purify DNA fragments from agarose gel</td>
		</tr>
		<tr>
			<td>Tris</td>
			<td>Aladdin, China</td>
			<td>T110601</td>
			<td>To prepare 1x TAE buffer</td>
		</tr>
		<tr>
			<td>TRIzol reagent</td>
			<td>Invitrogen, USA</td>
			<td>15596026</td>
			<td>To extract mitochondiral RNA.</td>
		</tr>
		<tr>
			<td>Universal DNA purification kit</td>
			<td>Tiangen Biotech, China</td>
			<td>DP214</td>
			<td>To recover linearized plastmids from the restriction enzyme digestion reaction</td>
		</tr>
		<tr>
			<td>Xylene cyanol FF</td>
			<td>Sigma-aldrich, USA</td>
			<td>X4126</td>
			<td>For preparation of loading buffer for agarose gel electrophoresis</td>
		</tr>
	</tbody>
</table>

discrimintion and mapping of the primary and processed transcripts in maize mitochondrion using a circular rt-pcr-based strategy

In plant mitochondria, some steady-state transcripts have 5&#39; triphosphate derived from transcription initiation (primary transcripts), while the others contain 5&#39; monophosphate generated post-transcriptionally (processed transcripts). To discriminate between the two types of transcripts, several strategies have been developed, and most of them depend on presence/absence of 5&#39; triphosphate. However, the triphosphate at primary 5&#39; termini is unstable, and it hinders a clear discrimination of the two types of transcripts. To systematically differentiate and map the primary and processed transcripts stably accumulated in maize mitochondrion, we have developed a circular RT-PCR (cRT-PCR)-based strategy by combining cRT-PCR, RNA 5&#39; polyphoshpatase treatment, quantitative RT-PCR (RT-qPCR), and Northern blot. As an improvement, this strategy includes an RNA normalization step to minimize the influence of unstable 5&#39; triphosphate.
In this protocol, the enriched mitochondrial RNA is pre-treated by RNA 5&#39; polyphosphatase, which converts 5&#39; triphsophate to monophosphate. After circularization and reverse transcription, the two cDNAs derived from 5&#39; polyphosphatase-treated and non-treated RNAs are normalized by maize 26S mature rRNA, which has a processed 5&#39; end and is insensitive to 5&#39; polyphosphatase. After normalization, the primary and processed transcripts are discriminated by comparing cRT-PCR and RT-qPCR products obtained from the treated and non-treated RNAs. The transcript termini are determined by cloning and sequencing of the cRT-PCR products, and then verified by Northern blot.
By using this strategy, most steady-state transcripts in maize mitochondrion have been determined. Due to the complicated transcript pattern of some mitochondrial genes, a few steady-state transcripts were not differentiated and/or mapped, though they were detected in a Northern blot. We are not sure whether this strategy is suitable to discriminate and map the steady-state transcripts in other plant mitochondria or in plastids.

Estimation of mitochondrial RNA circularization efficiency
In a previous study, both total and mitochondrial RNAs were used for cRT-PCR mapping of mitochondrial transcript termini in Arabidopsis (Arabidopsis thaliana), and the two types of RNAs gave similar mapping results12. Initially, we also used total RNAs for cRT-PCR mapping of mitochondrial transcript termini in ma...

Watch this Scientific Journal Video about Discrimintion and Mapping of the Primary and Processed Transcripts in Maize Mitochondrion Using a Circular RT-PCR-based Strategy at JoVE.com

Discrimintion and Mapping of the Primary and Processed Transcripts in Maize Mitochondrion Using a Circular RT-PCR-based Strategy

We present a circular RT-PCR-based strategy by combining circular RT-PCR, quantitative RT-PCR, RNA 5&#39; polyphosphatase-treatment, and Northern blot. This protocol includes a normalization step to minimize the influence of unstable 5&#39; triphosphate, and it is suitable for discriminating and mapping the primary and processed transcripts stably accumulated in maize mitochondrion.

In plant mitochondria, some steady-state transcripts have 5&#39; triphosphate derived from transcription initiation (primary transcripts), while the ...

This protocol is used for mapping and discrimination of the primary and processed transcripts in maize mitochondria. This technique includes an RNA normalization step and it minimize the influence of unstable five prime triphosphate that hinders a clear discrimination between the primary and the processed transcripts. Besides maize mitochondria, this method could be applied to plastids and other plant mitochondria.Start by collecting 20 grams of developing kernels into a 50 milliliter tube on ice. Transfer the kernels to a pre-cooled mortar and add 10 to 20 milliliters of ice cold extraction buffer. Grind the kernels completely, add more extraction buffer and filter the ground tissue through two layers of filter cloth.Centrifuge the filtrate at 8, 000 times G for 10 minutes. Then transfer the supernatant to a new tube and discard the filtrate. Centrifuge the tube at 20, 000 times G for 10 minutes.Pour off the supernatant and re-suspend the pellet in six milliliters of wash buffer. Aliquot the suspension into five 1.5 milliliter RNase-free tubes and centrifuge them at 14, 000 times G for five minutes. Discard the supernatant and freeze the mitochondrial pellet in liquid nitrogen.Extract mitochondrial RNA using commercial reagents according to the manufacturer's instructions and set up an RNA five prime polyphosphaatase treatment. Then, recover the RNA with an RNA purification kit. The reagent used for RNA extraction is hazardous.Always work with it in a fume hood and always wear lab coat and gloves. Prepare two circularization reactions using the same amounts of five prime polyphosphatase treated and non-treated mitochondrial RNA's. Incubate both reactions at 16 degrees celsius for 12 to 16 hours and then recover the self-ligated RNA's with a previously used RNA purification kit.Start by preparing a primer mixture with an equal ratio of 26 SCRT and up to seven other reverse transcription primers. Assemble two reverse transcription reaction systems according to the manuscript directions and incubate them at 42 degrees celsius for 50 minutes. To normalize the cDNA, prepare two PCR reactions with the same volume of cDNA's from the five prime polyphosphatase treated or non-treated RNA's and run the reaction under thermocycling conditions described in the manuscript.Normalization by 26S mature rRNA is a key step to discriminate between the primary and the processed transcripts. Compare the abundance of the two PCR products and if necessary, optimize the normalization by adjusting the amounts of template cDNA's. Prepare pairs of PCR reactions with appropriate volumes of normalized cDNA's and a pair of divergent primers like in the five prime to three prime junction of the target transcripts and perform PCR according to the manuscript directions.Then, use a gel DNA recovery kit to isolate the prominent bands that are amplified by nested PCR. Clone the gel purified PCR products into a vector using standard techniques and perform colony PCR to select positive clones containing the target inserts. Sequence the PCR products and align the sequencing data with the maize mitochondrial genome using the basic local alignment search tool or BLAST.Choose the organism maize and search database nucleotide collection. Find the five prime to three prime junction of the circularized transcript and determine the positions of the five prime and three prime transcript termini. Amplify the DNA fragment used to prepare the RNA probe and clone it to the previously used vector which contains a T7 promoter, 17 base pairs upstream of the insertion site.Label the RNA probes with dig-11-UTP and perform RNA hybridization using commercial kits. Discriminate the primary and processed five prime ends by comparing the circular RT-PCR products obtained from the normalized five prime polyphosphatase treated and non-treated RNA's. Then, design quantitative RT-PCR primers based on the mapping results and verify the discrimination results with RT-PCR.The COX-2 gene was used as an example to demonstrate the mapping and discrimination of maize mitochondrial transcripts with the circular RT-PCR based strategy. The normalized cDNA's were used as templates for PCR, which resulted in two prominent bands amplified from the five prime polyphosphatase treated sample. The two bands were named COX-2 one and two recovered from the gel and cloned into vectors.Colony PCR results showed that the positive clones contain inserts with variable size which implies that the termini have heterogeneous five prime or three prime termini. The sequencing results showed that COX-2 one and two have the same three prime ends at 39 nucleotides downstream of the stop codon while their five prime termini are located at 992 to 1, 030 and 1, 276 to 1, 283 nucleotides upstream of the start codon, respectively. RNA gel blot hybridization was performed to verify the circular RT-PCR mapping results and two major bands with sizes similar to COX-2 one and two were detected.Another pair of divergent primers were designed to amplify the COX-2 three and four bands detected with the RNA gel blot but not with first time circular RT-PCR. The quantitative RT-PCR results confirmed that COX-2 one, two, three and four were sensitive to five prime polyphosphatase and that they had primary five prime termini. Primer extension analysis could be performed to verify the five prime mapping results although it's not included in this protocol.

discrimintion-mapping-primary-processed-transcripts-maize

Research

JoVE Journal

Genetics

6.1K vues.  South China Agricultural University. Ce protocole est utilisé pour la cartographie et la discrimination des transcriptions primaires et traitées dans les mitochondries de maïs. Cette technique comprend une étape de normalisation de l’ARN et elle minimise l’influence de cinq triphosphates premiers instables qui entravent une discrimination claire entre les transcriptions primaires et traitées. Outre les mitochondries de maïs, cette méthode pourrait être appliquée aux plastides et autres mitochondries végétales....