Diploid yeast, like most eukaryotes, can undergo meiotic differentiation to form haploid gametes. Meiotic differentiation and cell growth (proliferation)are mutually exclusive programs, and in yeast the switch between growth and meiosis is controlled by nutritional signals. The signaling pathways that mediate nutritional controls on meiotic initiation fall into three broad classes: those that respond to nutrient starvation, those that respond to non-fermentable carbon sources, and those that respond to glucose. At the onset of meiosis, nutritional signaling pathways converge on transcriptional regulation of two genes: IME1, which encodes a transcription factor;and IME2, which encodes a protein kinase. Transcription of IME1 and IME2 trigger initiation of meiosis, and the expression of these two genes is linked with one other, with expression of later meiotic genes and with early meiotic events such as DNA replication. In addition, the signaling pathways that control IME1 and IME2expression are themselves integrated through a variety of mechanisms. Thus the signal network that controls the switch from growth to meiotic differentiation provides a signaling code that translates different combinations of extracellular signals into appropriate cellular responses.

In the budding yeast Saccharomyces cerevisiae, the cell division cycle and sporulation are mutually exclusive cell fates; glucose, which stimulates the cell division cycle, is a potent inhibitor of sporulation. Addition of moderate concentrations of glucose (0.5%) to sporulation medium did not inhibit transcription of two key activators of sporulation, IME1 and IME2, but did increase levels of Sic1p, a cyclin-dependent kinase inhibitor, resulting in a block to meiotic DNA replication. The effects of glucose on Sic1p levels and DNA replication required Grr1p, a component of the SCF Grr1p ubiquitin ligase. Sic1p is negatively regulated by Ime2p kinase, and several observations indicate that glucose inhibits meiotic DNA replication through SCF Grr1p -mediated destruction of this kinase. First, Ime2p was destabilized in the presence of glucose, and this turnover required Grr1p, a second component of SCF Grr1p , Cdc53p, and an SCF Grr1p -associated E2 enzyme, Cdc34p. Second, Ime2p-ubiquitin conjugates were detected under conditions of rapid Ime2p turnover, and conjugation of Ime2p to ubiquitin required GRR1. Third, a mutant form of Ime2p (Ime2 ⌬PEST ), in which a putative Grr1p-interacting sequence was deleted, was more stable than wild-type Ime2p. Finally, expression of the IME2⌬PEST allele bypassed the block to meiotic DNA replication caused by 0.5% glucose. In addition, Grr1p is required for later events in sporulation independently of its role in Ime2p turnover.

Uracil-DNA glycosylase (UDG), a key highly conserved DNA repair enzyme involved in uracil excision repair, was discovered in Escherichia coli . The Bacillus subtilis bacteriophage, PBS-1 and PBS-2, which contain dUMP residues in their DNA, express a UDG inhibitor protein, Ugi which binds to UDG very tightly to form a physiologically irreversible complex. The X-ray analysis of the E. coli UDG ( Ec UDG)-Ugi complex at 3.2 A resolution, leads to the first structure elucidation of a bacterial UDG molecule. This structure is similar to the enzymes from human and viral sources. A comparison of the available structures involving UDG permits the delineation of the constant and the variable regions of the molecule. Structural comparison and mutational analysis also indicate that the mode of action of the enzyme from these sources are the same. The crystal structure shows a remarkable spatial conservation of the active site residues involved in DNA binding in spite of significant differences in the structure of the enzyme-inhibitor complex, in comparison with those from the mammalian and viral sources. Ec UDG could serve as a prototype for UDGs from pathogenic prokaryotes, and provide a framework for possible drug development against such pathogens with emphasis on features of the molecule that differ from those in the human enzyme.

Background: IME1, which is required for the initiation of meiosis, is regulated by Cln3:Cdc28 kinase, which activates the G1‐to‐S transition, and Snf1 kinase, which mediates glucose repression. Here we examine the pathway by which Cln3:Cdc28p represses IME1 and the relationship between Cln3:Cdc28p and Snf1p in this regulation. Results: When wild‐type yeast cease growth, they express IME1 to moderate levels, intermediate between the low levels expressed during growth and the high levels expressed during sporulation. Moderate IME1 expression occurred in cln3Δ, cln1Δ cln2Δ, cdc28‐4 and swi6Δ mutants, even during growth. These mutants also induced IME1 expression more rapidly than the wild‐type. CLN3 required SWI6 and CLN2 to repress IME1 and IME2, but CLN1 was much less active than CLN2 in this repression. The phenotype of the cln3Δ snf1Δ double mutant indicated that Cln3:Cdc28p regulates IME1 independently of SNF1. Conclusion: Entry into meiosis involves two independent but sequential controls, which regulate IME1 via a three position switch: (i) during growth IME1 is repressed by the CLN3/SWI6/CLN2 pathway, (ii) once growth ceases, this repression is released and IME1 is expressed at moderate levels, and (iii) subsequently, nutritional conditions that activate Snf1p allow high IME1 expression.

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