Light-associated and Processing-dependent Protein Binding to 5′ Regions of rbcL mRNA in the Chloroplasts of a C₄ Plant

In amaranth, a C₄ dicotyledonous plant, the plastid rbcL gene (encoding the large subunit of ribulose-1,5-bisphosphate carboxylase) is regulated post-transcriptionally during many developmental processes, including light-mediated development. To identify posttranscriptional regulators of rbcL expression, three types of analyses (polysome heel printing, gel retardation, and UV cross-linking) were utilized. These approaches revealed that multiple proteins interact with 5′ regions of rbcL mRNA in light-grown, but not etiolated, amaranth plants. Light-associated binding of a 47-kDa protein (p47), observed by UV cross-linking, was highly specific for the rbcL 5′ RNA. Binding of p47 occurred only with RNAs corresponding to mature processed rbcL transcripts (5′-untranslated region (UTR) terminating at -66); transcripts with longer 5′-UTRs did not associate with p47 in vitro. Variations in the length of the rbcL 5′-UTR were found to occur in vivo, and these different 5′ termini may prevent or enhance light-associated p47 binding, possibly affecting rbcL expression as well. p47 binding correlates with light-dependent rbcL polysome association of the fully processed transcripts in photosynthetic leaves and cotyledons but not with cell-specific rbcL mRNA accumulation in bundle sheath and mesophyll chloroplasts.