The intron-mediated gene regulation in Saccharomyces cerevisiae

Abstract

Sm-like (Lsm) proteins are critically involved in a variety of RNA-processing events, including splicing, post-transcriptional modification and RNA degradation in organisms that range from bacteria and archea to yeast and humans. In Sacchromyces cerevisiae, the proteins existing in at least two heteroheptameric ring complexes: Lsm1-Lsm7, which promotes mRNA degradation via decapping in the cytoplasm; and, Lsm2-Lsm8, which is required for mRNA splicing in facilitating U4/U6 snRNP in the nucleus. Despite extensive understanding of their function, little is known about the mechanisms that regulate expression of the LSM genes. By constructing a set of mutants that lacked the LSM7 gene, or its intron, or which expressed the intron but not the exons, the LSM7 intron was shown to modulate expression of other LSM genes in trans. The intron located at a separate locus in the ADE1 gene was able to affect expression of some LSM genes although the patterns of expression in cells grown on different carbon sources were not the same as those in the wild type. Sequences within the intron that regulate LSM genes were identified through sequential targeted mutagenesis. The data indicated that the splicing elements are required for full function of the intron. Moreover, a 24 nt region of the intron was found important in controlling the level of expression of the mature LSM7 transcript and other LSM genes in the response to growth on different carbon sources. Microarray analysis was also employed to determine the full extent of the regulatory effect of the intron. Deletion of just the LSM7 intron affected a large group of genes involved in mating in haploids, and a mating efficiency assay provided strong evidence of the requirement of LSM7 intron in mating regulation. However, this regulation was not exerted through control in pheromone production or sensitivity towards opposite mating-type pheromone. These data constitute a step towards understanding not only the regulation of LSM genes but also provide an example of a novel mechanism for gene regulation driven by an intron that can act in trans in a relatively simple eukaryote that lacks the machinery for gene silencing.