Biosynthesis of cytochrome f in Chlamydomonas reinhardtii: analysis of the pathway in gabaculine-treated cells and in the heme attachment mutant B6.

Chlamydomonas reinhardtii uses two c-type cytochromes for photosynthetic electron transfer: the thylakoid membrane-bound cytochrome f of the cytochrome b6f complex and the soluble cytochrome c6. Previously, a class of photosynthesis-minus, acetate-requiring mutants was identified which were deficient in both c-type cytochromes, and biochemical analyses of cytochrome c6 biosynthesis in these strains indicated that they were each blocked at the step of heme attachment to apocytochrome c6. In order to demonstrate that the deficiency in cytochrome f results from the same biochemical and genetic defect, cytochrome f biosynthesis was examined in the B6 mutant (a representative of this phenotypic class) and in spontaneous suppressor strains derived from B6. Pulse-radiolabeling experiments show that B6 synthesizes a form of cytochrome f that is rapidly degraded in vivo. This polypeptide is membrane associated and migrates with an electrophoretic mobility identical to that of standard apocytochrome f produced in vitro but slightly greater than that of standard holocytochrome f produced in vivo by wild-type cells. These findings suggest that the B6 strain is unable to convert apocytochrome f to holocytochrome f and that apocytochrome f is unstable in vivo. In the suppressed strains, accumulation of both holocytochrome f and holocytochrome c6 is restored. One suppressor mutation (strain B6R) displays uniparental inheritance whereas another (B6T3) displays Mendelian inheritance. In both cases, the three phenotypes, photosynthesis-plus, b6f+ and cyt c6+ co-segregate in genetic crosses. This study therefore confirms that the dual cyt b6f-/cytc6- deficiency in B6 results from a single mutation that affects a step in holocytochrome formation that is common to the biosynthetic pathways of both plastidic c-type cytochromes. The study also confirms that pre-apocytochrome f synthesis, processing and association with the membrane is not dependent on heme attachment. Synthesis of cytochrome f does, however, appear to be dependent on heme availability. In cells depleted of tetrapyrrole pathway intermediates by gabaculine treatment, cytochrome f synthesis was significantly reduced. Since gabaculine treatment did not affect the stability of cytochrome f nor the accumulation of cytochrome f-encoding transcripts, the reduction is attributed to post-transcriptional regulation of preapocytochrome f synthesis via a pathway that is sensitive to the availability of heme or a tetrapyrrole pathway intermediate.