TRANSCRIPTIONAL INITIATION SITES IN SORGHUM MITOCHONDRIAL-DNA INDICATE CONSERVED AND VARIABLE FEATURES

Transcriptional initiation and processing was examined for three sorgh um mitochondrial DNA genes (atp6-1, atp6-2, urf209) and two open readi ng frames (orf265/130, orf107) to characterize sequences associated wi th initiation and other transcriptional strategies for this species. T he 5' termini of ten transcripts were determined by primer extension, and mtRNA was capped with guanylyl transferase and annealed to anti-se nse riboprobes to identify transcriptional initiation regions. Eight t ranscript termini were suitable substrates for guanylyl transferase, i ndicating the presence of one (atp6-1, atp6-2, urf209), two (orf265/13 0), or three (orf107) promoters for the five examples. The majority of the putative promoters were associated with single primer-extension t ermini, while two examples exhibited two transcript-initiation sites w ithin the promoter. Four examples were characterized by initiated tran scripts without subsequent processing, indicating that processing is n ot obligatory. Each of the putative promoter regions included signific ant A/T-rich 5' regions, consistent with previous examples, but four e xceptions to a consensus core YRTA sequence were identified. The anoma lies (AATA, CTTA) suggest plasticity in the primary structure of the c ore region of higher-plant mitochondrial DNA promoters.