CSP41, A SEQUENCE-SPECIFIC CHLOROPLAST MESSENGER-RNA BINDING-PROTEIN,IS AN ENDORIBONUCLEASE

Correct 3' processing of chloroplast precursor mRNAs (pre-mRNAs) requi res a stem-loop structure within the 3' untranslated region, In spinac h, a stable 3' stem-loop-protein complex has been shown to form in vit ro between petD pre-mRNA, encoding subunit IV of the cytochrome b(6)/f complex, and chloroplast proteins. This complex contains three chloro plast stem-loop binding proteins (CSPs), namely, CSP29, CSP41, and CSP 55. Here, we report the purification of CSP41 and cloning of the csp41 gene and show that CSP41 is encoded by a single nuclear gene. Charact erization of bacterially expressed CSP41 demonstrates that this protei n binds specifically to the 3' stem-loop structure and a downstream AU -rich element of petD pre-mRNA and that its binding affinity is enhanc ed by associating with CSP55. Our data also show that CSP41 has substa ntial nonspecific endoribonuclease activity. These data suggest that C SP41 could be involved in 3' processing of petD pre-mRNA and/or in RNA degradation. The fact that different reaction conditions favor RNA bi nding over ribonuclease activities suggests a possible mode of in vivo regulation.