Higher plant mitochondrial DNA expression : 1. Variant expression of the plant mitochondrial open reading frame, ORF25, in B37N and B73N maize lines.

In studying the process of mitochondrial transcription, mutants that show altered gene expression as evidenced from transcript pattern differences are a valuable resource. However, such mutants are difficult to find since changes in mitochondrial gene expression will most likely be lethal. Several laboratories have been investigating cytoplasmic male-sterile mutants in maize and have reported changes in transcription patterns due to nuclear background influences on the complex chimeric gene region TURF-2H3 in T-cms. There have been no reports of altered transcription patterns for N cytoplasm that can be attributed to nuclear background differences. Through a Northern hybridization analysis of ORF25 transcription in a number of N lines, we reported invariant expression of this region. Subsequently, we have discovered a line B37N, which shows the presence of a single ORF25-specific transcript of 3,400 nucleotides, in contrast to the transcript sizes of 3,400, 2,300 and 1,600 displayed by most of the cytoplasms we have examined. Experiments presented in this communication demonstrate that the differences in the B37N, ORF25 transcript pattern map to the 5' flanking sequences of the reading frame. Using restriction enzyme mapping and Southern hybridization analysis, no detectable differences were found in the transcription unit structure for this reading frame in B37N and B73N, which shows the standard, three-transcript pattern. Analysis of nuclear background influences indicates that the transcript patterns for this open reading frame are dependent on nuclear background. These data are presented in part 2 of this study.