A 43 KD LIGHT-REGULATED CHLOROPLAST RNA-BINDING PROTEIN INTERACTS WITH THE PSBA 5'-NON-TRANSLATED LEADER RNA

Expression of the chloroplast psbA gene coding for the D1 protein of P hotosystem II is subject to regulation at different levels in higher p lants, including control of mRNA accumulation and translation. In dico ts, the conserved 5' non-translated leader (5'-UTR) of the psbA mRNA i s sufficient to direct the light-dependent translation of the D1 prote in. In this report we show that the psbA mRNA 5'-UTR forms a stem-loop structure and binds a 43 kD chloroplast protein (43RNP). Binding of t he 43RNP is sensitive to competition with poly(U), but insensitive to high concentrations of tRNA, the RNA homopolymers poly(A), poly(G), po ly(C), or poly(A):poly(U) as a double-strand RNA. The 43RNP does not b ind efficiently to the psbA mRNA 3' non-translated region, although th e RNA sequence is U-rich and folds into a stem-loop. A deletion mutant of the psbA 5'-UTR RNA in which 5' sequences of the stem-loop are rem oved does not affect 43RNP binding. Together, these properties suggest that the 43RNP binds most effectively to a specific single-strand U-r ich sequence preceding the AUG start codon in the psbA mRNA. Binding o f the 43RNP is not detectable in plastid protein extracts from 5-day-o ld dark-grown seedlings, but is detectable in light-grown seedlings as well as mature plants in the light and after shifted to the dark. The 43RNP is therefore a candidate for a regulatory RNA-binding protein t hat may control the accumulation and/or translation of the psbA mRNA d uring light-dependent seedling development.