THE MAIZE RNA-BINDING PROTEIN, MA16, IS A NUCLEOLAR PROTEIN LOCATED IN THE DENSE FIBRILLAR COMPONENT

A developmentally and environmentally regulated gene in maize, MA16, encoding an RNA-binding protein that binds preferentially to uridine and guanosine-rich RNAs has previously been described. To gain some insight into the function of MA16 the distribution of MA16 mRNA and protein during maize development was investigated using in situ hybridization, RNA and protein gel blot analysis and immunocytochemistry. The results show that MA16 is expressed throughout development of the embryo and seedling in different tissues and at different levels. The level of MA16 mRNA is higher in developing and expanding structures such as the root elongation zone and young leaves. After stress treatment MA16 mRNA increases in total and polysomal RNA, but no significant change in the level of the protein was detected. MA16 is a non-ribosomal nucleolar protein. Using immunoelectron microscopy the MA16 protein has been located in the dense fibrillar component and to a lesser extent in the granular component of the nucleolus. It was found that MA16 contains the conserved sequence motifs R(G)(n)Y(G)(n)R and RR(E/D)(G)(n)Y(G)(n) repeated in the C-terminal of the molecule that conforms imperfectly to the GAR motif proposed for nucleolar proteins. In light of these results the stress regulation of MA16 and a likely role for this protein in pre-rRNA processing and/or ribosome assembly is discussed.