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The Chlamydomonas reinhardtii chloroplast mutants 18-SB and 18-7G lack both the chloroplast-encoded large subunit and nuclear-encoded small subunit of the chloroplast enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (EC 18.104.22.168). A chloroplast intergenic-suppression model has been postulated to account for the genetic instability of 18-5B revertants. Here, we have determined the molecular basis of the 18-SB and 18-7G mutants. They contain nonsense mutations close to the 3' and 5' ends of their large-subunit genes, respectively. Puls-chase experiments revealed that the 18-SB mutant produces a truncated large subunit that is unstable. In connection with previous experiments, this work identifies nonsense suppression in the chloroplast. Small subunits are also synthesized but then degraded in the mutants. Thus, the coordinated absence of subunits is achieved through degradation of the small subunit in the specific absence of the large subunit.