Processing of the psbA 5 ' untranslated region in Chlamydomonas reinhardtii depends upon factors mediating ribosome association

The 5' untranslated region of the chloroplast psbA mRNA, encoding the D1 pr otein, is processed in Chlamydomonas reinhardtii. Processing occurs just up stream of a consensus Shine-Dalgarno sequence and results in the removal of 54 nucleotides from the 5' terminus, including a stem-loop element identif ied previously as an important structure for D1 expression. Examination of this processing event in C. reinhardtii strains containing mutations within the chloroplast or nuclear genomes that block psbA translation reveals a c orrelation between processing and ribosome association. Mutations within th e 5' untranslated region of the psbA mRNA that disrupt the Shine-Dalgarno s equence, acting as a ribosome binding site, preclude translation and preven t mRNA processing. Similarly, nuclear mutations that specifically affect sy nthesis of the D1 protein specifically affect processing of the psbA mRNA. In vitro, loss of the stem-loop element does not prohibit the binding of a message-specific protein complex required for translational activation of p sbA upon illumination. These results are consistent with a hierarchical mat uration pathway for chloroplast messages, mediated by nuclear-encoded facto rs, that integrates mRNA processing, message stability, ribosome associatio n, and translation.