Translation of the chloroplast-encoded psbD mRNA is arrested post-initiation in a nuclear mutant of Chlamydomonas reinhardtii

Nac1-18 is a nuclear mutant of the unicellular green alga Chlamydomonas rei nhardtii, previously shown to lack translation of the psbD mRNA (Kuchka et al. 1988). Here we show that nac1-18 is deficient in D2 synthesis, despite the association of the corresponding psbD mRNA with polyribosomes. psbD mRN A translation is blocked post-initiation in nac1-18, suggesting that a nucl ear gene is specifically required for elongation of this mRNA. A decrease i n psbA translation is also observed in nac1-18, as previously seen in other mutants lacking D2 synthesis (Erickson et al. 1986). Here we show that ass ociation of psbA mRNA with polyribosomes is reduced in nac1-18 as a result of the loss of D2 synthesis, suggesting a relationship between D2 accumulat ion and D1 translation initiation.