TCA1, a single nuclear-encoded translational activator specific for petA mRNA in Chlamydomonas reinhardtii chloroplast

We isolated seven allelic nuclear mutants of Chlamydomonas reinhardtii spec ifically blocked in the translation of cytochrome f a major chloroplast-enc oded subunit of the photosynthetic electron transport chain encoded by the petA gene. We recovered one chloroplast suppressor in which the coding regi on of petA was now expressed under the control of a duplicated 5' untransla ted region from another open reading frame of presently unknown function. S ince we also recovered 14 nuclear intragenic suppressors, we ended up with 21 alleles of a single nuclear gene we called TCA1 for translation of cytoc hrome b(6)f complex petA mRNA. The high number of TCA1 alleles, together wi th the absence of genetic evidence for other nuclear loci controlling trans lation of the chloroplast petA gene, strongly suggests that TCA1 is the onl y trans-acting factor. We studied the assembly-dependent regulation of cyto chrome f translation-known as the CES process-in TCA1-mutated contexts. In the presence of a leaky tca1 allele, we observed that the regulation of cyt ochrome f translation was now exerted within dic limits of the restricted t ranslational activation conferred by the altered version of TCA1 as predict ed if TCA1 was the ternary, effector involved in the CES process.