Abstract
The Saccharomyces serevisiae genome is much more compact than those of higher eukaryotes as less than 5% of the yeast genes contain introns; these introns have been retained over the course of evolution, but the function they serve is unclear. Ribosomal protein genes make up more than one third of the genes that contain introns, but transcribe 90% of the mRNA of intron-containing genes. We wanted to investigate whether the introns of ribosomal protein gene modulate gene expression. We compared endogenously intron-containing ribosomal protein genes to endogenous intronless ones by overexpressing the genes in high-copy plasmids and testing the toxicity of these with a plasmid loss assay. There does not seem to be any difference in the toxicity of overexpressing intron-containing genes versus intronless genes, the overexpression plasmids generally follow the same pattern of loss for both classes of ribosomal protein genes. Comparing the intron-containing and intron-deleted versions of the same gene shows that removing the intron of the major ribosomal protein unit (RPL16B and RPS17A) is more toxic to the cells than having the intron of these genes. This is supported by plate data where a growth defect was seen in the background strain lacking an endogenous intron of RPL17A. Adding the plasmid overexpressing 17 A cDNA made these cells sicker, suggesting that too much RPS17 protein is produced and its intron is needed for proper regulation of gene expression. The intron of the major subunit of RPS17 seems to downregulate gene expression, possibly through the proposed negative feedback mechanism that inhibits splicing when the protein is in excess.