TRANSLATION OF THE PSBA MESSENGER-RNA OF CHLAMYDOMONAS-REINHARDTII REQUIRES A STRUCTURED RNA ELEMENT CONTAINED WITHIN THE 5' UNTRANSLATED REGION

Translational regulation is a key modulator of gene expression in chlo roplasts of higher plants and algae. Genetic analysis has shown that t ranslation of chloroplast mRNAs requires nuclear-encoded factors that interact with chloroplastic mRNAs in a message-specific manner. Using site-specific mutations of the chloroplastic psbA mRNA, we show that R NA elements contained within the 5' untranslated region of the mRNA ar e required for translation. One of these elements is a Shine-Dalgarno consensus sequence, which is necessary for ribosome association and ps bA translation. A second element required for high levels of psbA tran slation is located adjacent to and upstream of the Shine-Dalgarno sequ ence, and maps to the location on the RNA previously identified as the site of message-specific protein binding. This second element appears to act as a translational attenuator that must be overcome to activat e translation. Mutations that affect the secondary structure of these RNA elements greatly reduce the level of psbA translation, suggesting that secondary structure of these RNA elements plays a role in psbA tr anslation. These data suggest a mechanism for translational activation of the chloroplast psbA mRNA in which an RNA element containing the r ibosome-binding site is bound by message-specific RNA binding proteins allowing for increased ribosome association and translation initiatio n. These elements may be involved in the light-regulated translation o f the psbA mRNA.