The psbC gene of Chlamydomonas reinhardtii encodes P6, the 43 kd photosystem II core polypeptide. The sequence of P6 is highly homologous to the corresponding protein in higher plants with the exception of the N‐terminal region where the first 12 amino acids are missing. Translation of P6 is initiated at GUG in C. reinhardtii. The chloroplast mutant MA16 produces a highly unstable P6 protein. The mutation in this strain maps near the middle of the psbC gene and consists of a 6 bp duplication that creates a Ser‐Leu repeat at the end of one transmembrane domain. Two nuclear mutants, F34 and F64, and one chloroplast mutant, FuD34, are unable to synthesize P6. All of these mutants accumulate wild‐type levels of psbC mRNA. The FuD34 mutation has been localized near the middle of the 550 bp 5′ untranslated region of psbC where the RNA can be folded into a stem‐loop structure. A chloroplast suppressor of F34 has been isolated that partially restores synthesis of the 43 kd protein. The mutation of this suppressor is near that of FuD34, in the same stem‐loop region. These chloroplast mutations appear to define the target site of a nuclear factor that is involved in P6 translation.

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To study the interaction of the nuclear and chloroplast genomes in the biogenesis of the photosynthetic apparatus, nuclear mutants of Chlamydomonas reinhardtii deficient in photosystem II (PSII) activity were analyzed. Two independently‐isolated, allelic nuclear mutants show a pleiotropic reduction in a set of functionally related PSII polypeptides. Immunoblot analysis reveals that the two mutants, nac‐1‐18 and nac‐1‐11, accumulate reduced amounts of the chloroplast‐encoded polypeptides P5 and P6 and are completely deficient in polypeptides D1 and D2. Polypeptides of the oxygen‐evolving and light‐harvesting complexes associated with PSII, however, are present at wild‐type levels. Analysis of mRNAs encoding PSII polypeptides from these mutants indicates that all messages are present, although some species, including the D2 message, are significantly elevated. When mutant cells are pulse‐labeled for 10 min with [14C]acetate, a greatly reduced amount of labeled D2 protein is observed, while all other PSII polypeptides are synthesized normally. These data indicate that the mutations present in nac‐1‐18 and nac‐1‐11 affect a nuclear gene whose product specifically controls the translation and/or degradation of the chloroplast‐encoded D2 polypeptide.

A set of nuclear mutants of C. reinhardtii were identified that specifically lack translation of the chloroplast-encoded psbA mRNA, which encodes the photosystem II reaction center polypeptide D1. Two of these mutants are deficient in the 47-kD member (RB47) of the psbA RNA-binding complex, which has previously been identified both genetically and biochemically as a putative translational activator of the chloroplast psbA mRNA. RB47 is a member of the poly(A)-binding protein family, and binds with high affinity and specificity to the 5′ untranslated region of the psbA mRNA. The results presented here confirm RB47's role as a message-specific translational activator in the chloroplast, and bring together genetic and biochemical data to form a cohesive model for light- activated translational regulation in the chloroplast.