EXTRAGENIC SUPPRESSORS OF SACCHAROMYCES-CEREVISIAE PRP4 MUTATIONS IDENTIFY A NEGATIVE REGULATOR OF PRP GENES

The PRP4 gene encodes a protein that is a component of the U4/U6 small nuclear ribonucleoprotein particle and is necessary for both spliceos ome assembly and pre-mRNA splicing. To identify genes whose products i nteract with the PRP4 gene or gene product, we isolated second-site su ppressors of temperature-sensitive prp4 mutations. We limited ourselve s to suppressors with a distinct phenotype, cold sensitivity, to facil itate analysis of mutants. Ten independent recessive suppressors were obtained that identified four complementation groups, spp41, spp42, sp p43 and spp44 (suppressor of prp4, numbers 1-4). spp41-spp44 suppress the pre-mRNA splicing defect as well as the temperature-sensitive phen otype of prp4 strains. Each of these spp mutations also suppresses prp 3; spp41 and spp42 suppress prp11 as well. Neither spp41 nor spp42 sup presses null alleles of prp3 or prp4, indicating that the suppression does not occur via a bypass mechanism. The spp41 and spp42 mutations a re neither allele- nor gene-specific in their pattern of suppression a nd do not result in a defect in pre-mRNA splicing. Thus the SPP41 and SPP42 gene products are unlikely to participate directly in mRNA splic ing or interact directly with Prp3p or Prp4p. Expression of PRP3-lacZ and PRP4-lacZ gene fusions is increased in spp41 strains, suggesting t hat wild-type Spp41p represses expression of PRP3 and PRP4. SPP41 was cloned and sequenced and found to be essential. spp43 is allelic to th e previously identified suppressor srn1, which encodes a negative regu lator of gene expression.