Stability of the chloroplast psbD mRNA encoding the D2 protein of the photosystem II reaction center is drastically decreased in the nuclear photosynthetic mutant nac2-26 of Chlamydomonas reinhardtii. Using biolistic transformation and genetic crosses we have introduced chimeric genes consisting of the psbD leader fused to a reporter gene into the chloroplast in both wild-type and mutant nuclear backgrounds. The chimeric message is destabilized in the latter, but not in the former case, indicating that the 74 nt psbD leader includes one of the target sites for psbD RNA degradation in the absence of wild-type NAC2 function. Increased instability of the psbD leader in mutant versus wild-type chloroplast lysates is also demonstrated in vitro and the primary cleavage sites have been mapped. The instability of the psbD RNA in the mutant correlates with the loss of binding of a 47 kDa protein to the psbD leader RNA, suggesting that this factor acts as message stabilizer in wild-type.