mRNAs for two ribosomal proteins are preferentially translated in the chloroplast of Chlamydomonas reinhardtii under conditions of reduced protein synthesis

Abstract

Two mutants of the green alga Chlamydomonas reinhardtii, one deficient in the small subunit of the chloroplast ribosome and the other having chloroplast ribosomes with reduced function under certain conditions, show a characteristic syndrome of photosynthetic defects resulting from reduced chloroplast protein synthesis. These include subnormal levels of ribulose 1,5-bisphosphate carboxylase (Rubisco), reduced Hill reaction activity, diminished capacity to fix CO2, and abnormal thylakoid stacking. However, these mutants accumulate normal appearing chloroplast ribosome monomers or large subunits containing normal ribosomal protein components. In this paper, we demonstrate that pulse-labeled cells of these mutants synthesize two large subunit chloroplast ribosomal proteins at about 60% of the wild-type rate, whereas Rubisco large subunit (LSU) and the alpha subunit of CF1 are made at only 4 to 8% of the wild-type rate. No difference in the rate of turnover between ribosomal proteins and Rubisco LSU in mutant and wild-type cells was observed during a subsequent 60 min chase. Differences between the mutants and wild-type cells in the relative synthesis rates of these proteins were not reflected in the relative levels of mRNA (either hybridizable or in vitro translatable). In aggregate, these data suggest that C. reinhardtii preferentially translates chloroplast ribosomal protein mRNAs under conditions of reduced total chloroplast protein synthesis.

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

“…It is noteworthy that rps12 precursor transcripts detected in A31 were completely absent in the RR5 strain. This is due to the fact that for replacing the authentic rps12 gene with the psbD59-rps12 construct, the aadA cassette used for selecting the transformants was inserted in opposite orientation between rps12 and the upstream psaJ gene (Liu et al, 1989). It is thus likely that in this strain the maturation of other precursor transcripts of rps12 containing upstream sequences is altered.…”

“…Analysis of some of these mutants showed that under conditions of reduced chloroplast protein synthesis, ribosomal proteins are preferentially translated in comparison to proteins involved in photosynthesis (Liu et al, 1989). As one-third of chloroplast ribosomal proteins are synthesized on chloroplast ribosomes, this response may slow the permanent loss of chloroplast ribosomes.…”

Nuclear Mutants of Maize with Defects in Chloroplast Polysome Assembly Have Altered Chloroplast RNA Metabolism

“…Analysis of two Chlamydomonas mutants with defects in chloroplast translation led to the hypothesis that ribosomal protein mRNAs may be preferentially translated under conditions of reduced chloroplast protein synthesis (Liu et al, 1989). One of the mutants used in those studies, cr-7, resembled hcf7 in that it was deficient, specifically, in the small ribosomal subunit (Harris et al, 1974).…”

Translational Regulation of Chloroplast Genes

“…Two mutant strains deficient in chloroplast protein synthesis were utilized: 1) the nuclear ac-20 cr-1 double mutant which results in a permanent deficiency of chloroplast ribosome monomers (18), and 2) the chloroplast 16 S rDNA mutant, spr-u-1-27-3, which when grown mixotrophically in the presence of spectinomycin, accumulates nearly wild type levels of chloroplast ribosomes, but is deficient in chloroplast protein synthesis (9). Mixotrophically grown cells of ac-20 cr-1 were found to be severely deficient in the chloroplast encoded D1, LSU, and ␤ photosynthetic proteins, but accumulated nearly wild type levels of chloroplast encoded r-proteins S7 and S12 (Fig.…”

“…When chloroplast protein synthesis is reduced, C. reinhardtii preferentially translates mRNAs for chloroplast-encoded ribosomal proteins (r-proteins), while translation of mRNAs for photosynthetic proteins is severely diminished (9). Furthermore mRNAs for chloroplast-encoded r-proteins appear to be constitutively transcribed, accumulate early during chloroplast development in land plants and are preferentially loaded on polysomes (10,11).…”