Post-transcriptional defects in tobacco chloroplast mutants lacking the cytochrome b(6)/f complex

A variety of post-transcriptional mechanisms govern the synthesis and assembly of photosynthetic protein complexes in chloroplasts. To test whether such mechanisms are conserved between photosynthetic algae and vascular plants, we have interrupted the chloroplast petA, petB and petD genes of tobacco, which encode three subunits of the cytochrome b(6)/f complex, and compared our results to those previously obtained with Chlamydomonas reinhardtii. As expected, the mutants exhibited high chlorophyll fluorescence, consistent with the loss of a functional cytochrome b(6)/f complex. Unlike the corresponding mutants of Chlamydomonas, however, cytochrome f was barely detectable in the Delta petB or Delta petD mutants. The amounts of petB- and petD-containing mRNAs were reduced in the mutants compared to wild-type plants, but the remaining mRNA was normally associated with polysomes. In contrast, there was a decrease in polysome association of the polycistronic petA mRNA in the Delta petB and Delta petD mutants, suggesting that the synthesis of cytochrome f may be decreased in the absence of cytochrome b(6) or SUIV. These results are discussed in light of the translational autoregulation model that has been proposed for cytochrome b(6)/f complex assembly in Chlamydomonas.