Nuclear mutations specifically affect the synthesis and/or degradation of the chloroplast-encoded D2 polypeptide of photosystem II in Chlamydomonas reinhardtii

Abstract

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.

Evidence for Nuclear Control of the Expression of the atpA and atpB Chloroplast Genes in Chlamydomonas

“…A similar overaccumulation of untranslated transcripts has been previously reported for mutants of Chlamydomonas nad-11 and nad-18, which do not synthesize polypeptide D2, a polypeptide of the PSll core complex (Kuchka et al, 1988). This is in contrast with fscherichia coli where untranslated mRNAs are rapidly degraded (Cole and Nomura, 1986).…”

“…Genetic analysis of the mutant strains showed that the three nuclear mutations were in three different nuclear genes dispersed on the nuclear genome. Thus, our study adds to the growing body of experimental evidence that a number of chloroplast genes are expressed under nuclear controls operating at the level of transcript stability (Jensen et al, 1986;Kuchkaetal., 1989;Sieburthetal., 1991) or at the level of translation (Kuchka et al, 1988;Rochaix et al, 1989).…”

“…would be required for the stabilization and for the translation of mRNAs of chloroplqst-encoded PSll subunits (Jensen et al, 1986;Kuchka et al, 1988;Kuchka et al, 1989;Rochaix et al, 1989;Sieburth et al, 1991). The nuclear control on the mRNA processing of one of the two main PSI subunits has also been extensively studied (Choquet et al, 1988;Goldschmidt-Clermont et al, 1990).…”

Repression of Essential Chloroplast Genes Reveals New Signaling Pathways and Regulatory Feedback Loops inChlamydomonas

“…Alternatively a ribosome-associated nucleolytic activity could degrade chloroplast mRNAs once they are translated but no longer when translation or transcription is compromised. Increased mRNA levels have also been observed in several cases in mutants of C. reinhardtii deficient in translation of psbA (Kuchka et al, 1988), atpA (Drapier et al, 1992), and psaB mRNA (Xu et al, 1993). Similar feedback loops have also been detected in bacteria for ribosomal operons, including tufA (Young and Furano, 1981), suggesting that these regulatory mechanisms have been conserved during the evolution of chloroplasts.…”

Chloroplast RNA Stability in Chlamydomonas: Rapid Degradation of psbB and psbC Transcripts in Two Nuclear Mutants.

“…mutant 6.2z5. Destabilization of other photosystem II components because of the loss of one integral membrane component has been documented for mutants whose primary lesions affect polypeptide 5, D1 (Jensen et al, 1986), D2 (Schmidt et al, 1987;Kuchka et al, 1988), and other mutants affecting synthesis of polypeptide 6 (Rochaix et al, 1989). Although polypeptide 6 synthesis is not required for translation of mRNAs for other photosystem II subunits, it is necessary for the stabilization of the photosystem II polypeptides in the thylakoid membrane.…”