Inversions in the chlamydomonas chloroplast genome suppress a petD 5' untranslated region deletion by creating functional chimeric mRNAs

Abstract

FUD6 is a non-photosynthetic Chlamydomonas mutant that lacks the cytocrome b6/f complex, due to a 236 bp deletion that removes the promoter and part of the 5' untranslated region (UTR) of the chloroplast petD gene, which encodes subunit IV of the complex. Two photosynthetic revertants of FUD6 that synthesized wild-type levels of subunit IV were found to contain related inversions of the chloroplast genome that resulted from recombination between small inverted repeats. These inversions created a functional chimeric petD gene that includes the promoter and part of the 5' UTR of the newly identified ycf9-psbM transcription unit, fused to the petD 5' UTR upstream of the FUD6 deletion. Accumulation of the ycf9-psbM dicistronic transcript was disrupted in the revertants, but monocistronic psbM mRNA accumulated normally. The FUD6 revertants demonstrate the ability of the Chlamydomonas chloroplast genome to undergo a large inversion without a deleterious effect on chloroplast function, reminiscent of events that have led to the evolutionary divergence of chloroplast genomes.