A nucleus-encoded suppressor defines a new factor which can promote petD mRNA stability in the chloroplast of Chlamydomonas reinhardtii

Mutations in the Chlamydomonas reinhardtii nuclear gene MCD1 specifically d estabilize the chloroplast petD mRNA, which encodes subunit IV of the cytoc hrome b(6)/f complex. The MCD1 gene product is thought to interact with the mRNA 5' end to protect it from degradation by a 5' --> 3' exoribonuclease and may also have a role in translation initiation. Here we report the isol ation and characterization of a semidominant, allele-specific, nucleus-enco ded suppressor of the mcd1-2 mutation. The suppressor mutation, which defin es a new locus MCD2, allows accumulation of 10% of the wildtype level of pe tD mRNA and as much as 50% of the wild-type subunit IV level. Taken togethe r, these results suggest the suppressor mutation restores photosynthetic gr owth by stabilizing petD mRNA. In addition, it may promote increased transl ational efficiency, an inference supported by direct measurements of the su bunit IV synthesis rate. Thus, both MCD1 and MCD2 may participate in both c hloroplast RNA stability and translation initiation.