During spoiulation of Saccharomyces cerevisiae, meiosis is followed by encapsulation of haploid nuclei within multilayered spore walls. Completion of the late events of the sporulation program requires the SPSl gene. This developmentally regulated gene, which is expressed as cells are nearing the end of meiosis, encodes a protein with homology to serine/threonine protein kinases. The catalytic domain of Spsl is 44% identical to the kinase domain of yeast Ste20, a protein involved in the pheromone-induced signal transduction pathway. Cells of a MATa./MATa. spsl/spsl strain arrest after meiosis and fail to activate genes that are normally expressed at a late time of sporulation. The mutant cells do not form refractile spores as assessed by phase-contrast microscopy and do not display the natural fluorescence and ether resistance that is characteristic of mature spores. Examination by electron microscopy reveals, however, that prospore-like compartments form in some of the mutant cells. These immature spores lack the cross-linked surface layer that surrounds wild-type spores and are more variable in size and number than are the spores of wild-type cells. Despite their inability to complete spore formation, spsl-arrested cells are able to resume mitotic growth on transfer to rich medium, generating haploid progeny. Our results suggest that the developmentally regulated Spsl kinase is required for normal progression of transcriptional, biochemical, and morphological events during the later portion of the sporulation program.

To identify genes important in fat-cell metabolism and development, we have screened Drosophila stocks carrying an engineered transposable element that can reveal the presence of nearby enhancer elements. We have identified those "enhancer-trap lines" that contain transposable P elements integrated near fat-cell specific enhancer elements. We anticipate that the genes associated with these enhancers will provide information concerning fat-cell function and serve as target genes for studying fat-cell specific gene expression. Furthermore, the identification of enhancer-trap lines active in the developing fat cell should provide an entry point into the molecular and genetic analysis of early fat-cell development. Analysis of two lines has revealed that the transcription factors svp, a steroid-hormone receptor, and Kr, a zinc-finger protein, are present in the fat body; these factors are likely to be involved in fat-cell gene expression. In two other lines, beta-galactosidase was detected in a subset of adepithelial cells that may be the precursors to the adult fat cell. And finally, in a single line transgene activity is present in the progenitor cells of the embryonic fat body. The genes associated with these enhancer-trap lines may be involved in fat-cell development.

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