Transcriptional initiation sites in sorghum mitochondrial DNA indicate conserved and variable features

Transcriptional initiation and processing was examined for three sorghum mitochondrial DNA genes (atp6-1, atp6-2, urf209) and two open reading frames (orf265/130, orf107) to characterize sequences associated with initiation and other transcriptional strategies for this species. The 5' termini of ten transcripts were determined by primer extension, and mtRNA was capped with guanylyl transferase and annealed to anti-sense riboprobes to identify transcriptional initiation regions. Eight transcript termini were suitable substrates for guanylyl transferase, indicating the presence of one (atp6-1, atp6-2, urf209), two (orf265/130), or three (orf107) promoters for the five examples. The majority of the putative promoters were associated with single primer-extension termini, while two examples exhibited two transcript-initiation sites within the promoter. Four examples were characterized by initiated transcripts without subsequent processing, indicating that processing is not obligatory. Each of the putative promoter regions included significant A/T-rich 5' regions, consistent with previous examples, but four exceptions to a consensus core YRTA sequence were identified. The anomalies (AATA, CTTA) suggest plasticity in the primary structure of the core region of higher-plant mitochondrial DNA promoters.