THE 5' LEADER OF A CHLOROPLAST MESSENGER-RNA MEDIATES THE TRANSLATIONAL REQUIREMENTS FOR 2 NUCLEUS-ENCODED FUNCTIONS IN CHLAMYDOMONAS-REINHARDTII

In the green alga Chlamydomonas reinhardtii, the nuclear mutations F34 and F64 have been previously shown to abolish the synthesis of the ph otosystem II core polypeptide subunit P6, which is encoded by the chlo roplast psbC gene. In this report the functions encoded by F34 and F64 are shown to be required for translation of the psbC mRNA, on the bas is of the finding that the expression of a heterologous reporter gene fused to the psbC 5' nontranslated leader sequence requires wild-type F34 and F64 alleles in vivo. Moreover, a point mutation in the psbC 5' nontranslated leader sequence suppresses this requirement for wild-ty pe F34 function. In vitro RNA-protein cross-linking studies reveal tha t chloroplast protein extracts from strains carrying the F64 mutation contain an approximately 46-kDa RNA-binding protein. The absence of th e RNA-binding activity of this protein in chloroplast extracts of wild -type strains suggests that it is related to the role of the F64-encod ed function for psbC mRNA translation. The binding specificity of this protein appears to be for an AU-rich RNA sequence motif.