The organization and expression of the Saccharomyces cerevisiae L4 ribosomal protein genes and their identification as the homologues of the mammalian ribosomal protein gene L7a

Abstract

A cDNA for the mouse ribosomal protein (rp) L7a, formerly called Surf-3, was used as a probe to isolate two homologous genes from Saccharomyces cerevisiae. The two yeast genes (L4-1 and L4-2) were identified as encoding S. cerevisiae L4 by 2D gel analysis of the product of the in vitro translation of hybrid-selected mRNA and additionally by direct amino acid sequencing. The DNA sequences of the two yeast genes were highly homologous (95%) over the 771 bp that encode the 256 amino acids of the coding regions but showed little homology outside the coding region. L4-1 differed from L4-2 by 7 out of the 256 amino acids in the coding region, which is the greatest divergence between the products of any two duplicated yeast ribosomal protein genes so far reported. There is strong homology between the mouse rpL7a/Surf-3 and the yeast L4 genes -57% at the nucleic acid level and also 57% at the amino acid level (though some regions reach as much as 80-90% homology). While most yeast ribosomal protein genes contain an intron in their 5' region both L4-1 and L4-2 are intronless. The mRNAs derived from each yeast gene contained heterogenous 5' and 3' ends but in each case the untranslated leaders were short. The L4-1 mRNA was found to be much more abundant than the L4-2 mRNA as assessed by cDNA and transcription analyses. Yeast cells containing a disruption of the L4-1 gene formed much smaller colonies than either wild-type or disrupted L4-2 strains. Disruption of both L4 genes is a lethal event, probably due to an inability to produce functional ribosomes.