Evolutionarily distinct classes of S27 ribosomal proteins with differential mRNA expression in rat hypothalamus

Using an in situ hybridization screen for cDNA clones of brain region-speci fic mRNAs, we isolated a rat transcript that encodes a ribosomal protein S2 7. Searching GenBank DNA databases, we found two S27 protein isoforms. One isoform, encoded by multiple genes, is extant in archaea and eukarya, but n ot bacteria. The second isoform appears to be recently evolved because it h as been identified only in mammals. Multiple transcripts encode each isofor m and exhibit different tissue expression patterns throughout rat brain and periphery, with abundant expression in the hypothalamus. In situ hybridiza tion studies revealed predominant expression of S27(1) in distinct hypothal amic nuclei, such as the paraventricular, supraoptic, suprachiasmatic, arcu ate, and circularis nuclei, whereas expression of S27(2) mRNA was discretel y expressed in select neurons of the periventricular and supraoptic nuclei. Combined with the genetic evidence that S27 has extraribosomal functions i n plants, the complexity of S27 biology observed here may suggest auxiliary functions for S27 proteins in the mammalian nervous system.