INVERSIONS IN THE CHLAMYDOMONAS CHLOROPLAST GENOME SUPPRESS A PETD 5'-UNTRANSLATED REGION DELETION BY CREATING FUNCTIONAL CHIMERIC MESSENGER-RNAS

FUD6 is a non-photosynthetic Chlamydomonas mutant that lacks the cytoc hrome b(6)/f complex, due to a 236 bp deletion that removes the promot er and part of the 5' untranslated region (UTR) of the chloroplast pet D gene, which encodes subunit IV of the complex. Two photosynthetic re vertants of FUD6 that synthesized wild-type levels of subunit IV were found to contain related inversions of the chloroplast genome that res ulted from recombination between small inverted repeats. These inversi ons created a functional chimeric petD gene that includes the promoter and part of the 5' UTR of the newly identified ycf9-psbM transciption unit, fused to the petD 5' UTR upstream of the FUD6 deletion. Accumul ation of the ycf9-psbM dicistronic transcript was disrupted in the rev ertants, but monocistronic psbM mRNA accumulated normally. The FUD6 re vertants demonstrate the ability of the Chlamydomonas chloroplast geno me to undergo a large inversion without a deleterious effect on chloro plast function, reminiscent of events that have led to the evolutionar y divergence of chloroplast genomes.