Mutations in two distinct nuclear genes, called NAC 1 and AC-115, of the unicellular green alga Chlamydomonas reinhardtii cause a specific and dramatic reduction in the synthesis of the chloroplast-encoded D2 polypeptide of Photosystem II. The psbD transcript which encodes the D2 protein is present in the mutant strains, but protein pulse-labeling and immunoprecipitation experiments demonstrate that the synthesis of the D2 protein does not occur normally in these cells. These phenotypes are suppressed by an extragenic nuclear suppressor isolated from a pseudorevertant of a nac 1 mutant. This suppressor is neither allele- nor gene-specific in its suppression and is able to overcome the effects of two different mutations in the NAC 1 gene, as well as a mutation in AC-115. The suppressor seems to be specific in its ability to remedy blocks in psbD mRNA translation in the chloroplast. It is not able to restore the translation of another chloroplast-encoded mRNA which is blocked by another nuclear mutation. The suppressor may identify a new nuclear gene specifically involved in the synthesis of the D2 protein in the chloroplast.