THE PETD GENE IS TRANSCRIBED BY FUNCTIONALLY REDUNDANT PROMOTERS IN CHLAMYDOMONAS-REINHARDTII CHLOROPLASTS

FUD6, a nonphotosynthetic mutant of Chlamydomonas reinhardtii, was pre viously found to be deficient in the synthesis of subunit IV of the cy tochrome b(6)/f complex, the chloroplast petD gene product (C. Lemaire , J. Girard-Bascou, F.-A. Wollman, and P. Bennoun, Biochim. Biophys. A cta 851:229-238, 1986). The lesion in FUD6 is a 236-bp deletion betwee n two 11-bp direct repeats in the chloroplast genome. It extends from 82 to 72 bp upstream of the 5' end of wild-type petD mRNA to 156 to 16 6 bp downstream of the 5' end. Thus, the deletion extends into the put ative promoter and 5' untranslated region of petD. No petD mRNA of the normal size can be detected in FUD6 cells, but a low level of a dicis tronic message accumulates, which contains the coding regions for subu nit IV and cytochrome f, the product of the upstream petA gene. petD t ranscriptional activity in FUD6 is not significantly altered from the wild-type level. This transcriptional activity was eliminated by petA promoter disruptions, suggesting that it originates at the petA promot er. We conclude that the petD-coding portion of most cotranscripts is rapidly degraded in FUD6, possibly following processing events that ge nerate the 3' end of petA mRNA. A chloroplast transformant was constru cted in which only the sequence from -81 to -2 relative to the major 5 ' end of the petD transcript was deleted. Although this deletion elimi nates all detectable petD promoter activity, the transformant grows ph ototrophically and accumulates high levels of monocistronic petD mRNA. We conclude that the petD gene can be transcribed by functionally red undant promoters. In the absence of a functional petD promoter, a lack of transcription termination allows the downstream petD gene to be co transcribed with the petA coding region and thereby expressed efficien tly.