RAT RL23A RIBOSOMAL-PROTEIN EFFICIENTLY COMPETES WITH ITS SACCHAROMYCES-CEREVISIAE L25 HOMOLOG FOR ASSEMBLY INTO 60-S SUBUNITS

The large subunit protein RL23a from rat liver ribosomes shows 62% seq uence identity with the primary rRNA-binding ribosomal protein L25 fro m Saccharomyces cerevisiae. In vitro binding studies indicated that bo th r-proteins are able to recognise the L25 binding site on yeast 25 S rRNA and its structural homologue on mammalian 28 S rRNA with equal e fficiency. To determine whether the two r-proteins are also functional ly equivalent in vivo, a single plasmid-borne copy of either the wild- type L25 gene or the RL23a cDNA, driven by the L25 promoter, was intro duced into a yeast strain in which the chromosomal L25 gene is under c ontrol of the glucose-repressible GALI-10 promoter. No difference in g rowth rate could be detected between the two types of transformants wh en cultured on glucose-based medium. In cells that co-express epitope- tagged versions of L25 and RL23a from single-copy genes, approximately 35% of the 60 S subunits contained the heterologous protein as determ ined by Western analysis. This value could be increased to 55% by over expressing RL23a using a multi-copy plasmid. These data demonstrate th at rat RL23a can act as a highly efficient substitute for its yeast co unterpart in the assembly of functional yeast ribosomes even in the pr esence of the endogenous L25 protein. (C) 1996 Academic Press Limited