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.

Open reading frame 4 (ORF4) of the potato virus X (PVX) genome encodes an 8K protein which is a part of the "triple gene block" and is known to play a role in the cell-to-cell movement of the virus in infected plants. To locate the 8K protein and further elucidate the mechanism of cell-to-cell transport of PVX, antibodies were raised against the 8K protein and used to localize this protein in PVX-infected tobacco and in transgenic potato plants expressing the 8K protein both by subcellular fractionation and by immunolabeling with colloidal gold. The results indicated that the 8K protein was localized to the cell wall.

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