Transcription of the S-2 maize mitochondrial plasmid

Mitochondria from S (male-sterile) and RU (male-fertile) maize cytoplasms contain both autonomous plasmids and copies of plasmid sequences integrated into the mitochondrial genome. The S-2 plasmid in S, which appears essentially the same as the R-2 plasmid in RU, contains two large open reading frames (ORFs) that may encode gene products required for plasmid maintainance and replication. Transcripts from both ORF1 and ORF2 are readily detected in cytoplasms having free plasmids. The 5' ends of these RNAs, which are transcribed from opposite ends of the plasmid, correspond to nucleotide position 32 within each of the 208 bp terminal inverted repeats (IRs). In Normal (fertile) and Revertant S (now fertile) cytoplasms that have only integrated copies of the S-2 sequence, transcripts from the S-2 ORFs are either present in greatly reduced amounts, or not detectable. The loss of transcriptional activity is correlated with deletions of, or nucleotide changes within, the IRs associated with the integrated S-2 ORFs. Evidence for transcription of non-plasmid DNA adjacent to integrated IRs suggests that both free-plasmid and integrated forms of the plasmid IR have promoter-like activity.