Target and specificity of a nuclear gene product that participates in mRNA3 '-end formation in Chlamydomonas chloroplasts

Chloroplast mRNA maturation is catalyzed by nucleus-encoded processing enzy mes. We previously described a recessive nuclear mutation (crp3) that affec ts 3'-end formation of several chloroplast mRNAs in Chlamydomonas reinhardt ii (Levy, H,, Kindle, K, L., and Stern, D, B, (1997) Plant Cell 9, 825-836) . In the crp3 background, atpB mRNA lacking a 3'-inverted repeat normally r equired for stability accumulates as a discrete transcript. The mutation al so affects the atpA gene cluster; polycistronic mRNAs with psbI or cemA 3'- ends accumulate to a lower level in the crp3 background. Here, we demonstra te that the crp3 mutation also alters S'-end formation of psbI mRNA and cem A-containing mRNAs. A novel 3'-end is formed in monocistronic psbI transcri pts, and this is the only terminus observed when the psbI 3'-untranslated r egion is fused to an aadA reporter gene. Accumulation of mRNAs with 3'-ends between cemA and atpH, which is immediately downstream, was reduced. Howev er, this sequence was not recognized as a 3'-end formation element in chime ric genes. The crp3 mutation was able to confer stability to three differen t atpB 3'-stem-loop-disrupting mutations that lack sequence similarity, but are located at a similar distance from the translation termination codon, We propose that the wild-type CRP3 gene product is part of the general 3' - -> 5' processing machinery.