The tobacco, human and two Arabidopsis genes for a highly conserved eukaryotic ribosomal protein (CEP52) contain introns. In each gene, one intron contains open reading frame which would allow the production of an altered form of the CEp52 protein (ALT52) from unspliced message. Based on immunoblots probed with CEP52 specific antiserum, it is proposed that translation of unspliced mRNA occurs. A putative ALT52 is expressed at high levels in rat tissues, but the protein is not ribosomally associated. ALT52 synthesis appears to be tissue specific and inversely related to the level of CEP52 present. Since CEP52 levels correspond to the amount of ribosomes present in rat tissues, the production of ALT52 may function to limit ribosomes production. These preliminary results suggests that CEP52 splicing may be a novel regulatory mechanism involved in controlling the levels of cytoplasmic ribosomes. The investigators will investigate these observations by characterizing the rat proteins that react with the CEP52 antibody and by sequencing a rat CEP52 genomic clone to determine if the gene contains introns as do the other characterized CEP52 genes. Additional antibodies will be generated to examine the possible production of ALT52 in the plant Arabidopsis. %%% Cellular ribosome levels are controlled by the need for the proteins that they synthesize. Rapidly growing cells, whether normal or cancerous, require elevated amounts of ribosomes compared to cells that are not as metabolically active. Despite significant advances, gaps remain in our understanding of how the assembly of two ribosomes from more that 80 proteins and four nucleic acids is coordinated and regulated. Two ribosomal proteins discovered in 1989 may have important roles in the regulation of ribosome assembly, since, an insufficient amount of either protein results in the decreased production of functional ribosomes. Recent studies suggest that one of these proteins may be regulated by editing the RNA copy of the genetic information once it is transcribed from DNA. This editing process occurs for many proteins, however, this case is unusual in that both the edited and unedited form of the RNA could be used to produce a protein. The proteins would be identical in some parts of their structure, but would significantly differ in size and probably function Preliminary evidence indicates that two forms of this protein are produced. The large version that would be produced from the unedited RNA is not found on ribosomes, and it is produced in the largest amounts by tissues that have few ribosomes. These studies will determine if the detected proteins are produced as the result of RNA editing of a common RNA precursor and if the editing process represents a previously unknown regulatory mechanism that controls ribosome levels.
