The Ink4a͞Arf locus encodes p16 Ink4a and p19 Arf and is among the most frequently mutated tumor suppressor loci in human cancer. In mice, many of these effects appear to be mediated by interactions between p19 Arf and p53, Ink4a͞Arf expression correlates with or precedes the emergence of cells expressing mutant p53. These data demonstrate that p19 Arf is an important part of the cellular defense mounted against transforming signals from the Abl oncoprotein and provide direct evidence that the p19 Arf -p53 regulatory loop plays an important role in lymphoma induction.

Transformation of pre-B cells by Abelson murine leukemia virus (Ab-MLV) involves a balance between positive, growth-stimulatory signals from the v-Abl oncoprotein and negative regulatory cues from cellular genes. This phenomenon is reflected by the clonal selection that occurs during Ab-MLV-mediated transformation in vivo and in vitro. About 50% of all Ab-MLV-transformed pre-B cells express mutant forms of p53 as they emerge from this process, suggesting that this protein may play an important role in the transformation process. Consistent with this idea, expression of p19(Arf), a protein whose function depends on the presence of a functional p53, is required for the apoptotic crisis that characterizes primary Ab-MLV transformants. To test the role of p53 in pre-B-cell transformation directly, we examined the response of Trp53(-/-) mice to Ab-MLV. The absence of p53 shortens the latency of Abelson disease induction but does not affect the frequency of cells susceptible to Ab-MLV-induced transformation. However, primary transformants derived from the null animals bypass the apoptotic crisis that characterizes the transition from primary transformant to fully malignant cell line. These effects do not require p21(Cip-1), a major downstream target of p53; however, consistent with a role of p19(Arf), transformants expressing mutant p53 and abundant p19 retain wild-type p19 sequences.

In Escherichia coli, repression of the aceBAK operon is mediated by the IclR protein. We used an in vitro oligonucleotide selection technique to determine the consensus recognition sequence for IclR. Mutational analysis confirmed the contribution of this sequence to repression in vivo and identified the ؊35 element of the promoter.

Yeast glycogen metabolism responds to environmental stressors such as nutrient limitation and heat shock. This response is mediated, in part, by the regulation of the glycogen metabolic genes. Environmental stressors induce a number of glycogen metabolic genes, including GPH1, which encodes glycogen phosphorylase. Primer extension analysis detected two start sites for GPH1, one of which predominated. Sequences upstream of these sites included a possible TATA element. Mutation of this sequence reduced GPH1 expression by a factor of 10 but did not affect start site selection. This mutation also did not affect the relative induction of GPH1 upon entry into stationary phase. Three candidates for stress response elements (STREs) were found upstream of the TATA sequence. Mutation of the STREs showed that they were required for regulation of GPH1 expression in early stationary phase, and in response to osmotic shock and heat shock. These elements appeared to act synergistically, since the intact promoter exhibited 30-fold more expression in stationary phase than the sum of that observed for each element acting independently. HOG1, which encodes a MAP kinase, has been implicated in control mediated by STREs. For GPH1, induction by osmotic shock depended on a functional HOG1 allele. In contrast, induction upon entry into stationary phase was only partially dependent on HOG1. Furthermore, the heat shock response, which can also be mediated by STREs, was independent of HOG1. These observations suggest that the GPH1 STREs respond to more than one pathway, only one of which requires HOG1.

GSY1 is one of the two genes encoding glycogen synthase in Saccharomyces cerevisiae. Both the GSY1 message and the protein levels increased as cells approached stationary phase. A combination of deletion analysis and site-directed mutagenesis revealed a complex promoter containing multiple positive and negative regulatory elements. Expression of GSY1 was dependent upon the presence of a TATA box and two stress response elements (STREs). Expression was repressed by Mig1, which mediates responses to glucose, and Rox1, which mediates responses to oxygen. Characterization of the GSY1 promoter also revealed a novel negative element. This element, N1, can repress expression driven by either an STRE or a heterologous element, the UAS of CYC1. Repression by N1 is dependent on the number of these elements that are present, but is independent of their orientation. N1 repressed expression when placed either upstream or downstream of the UAS, although the latter position is more effective. Gel shift analysis detected a factor that appears to bind to the N1 element. The complexity of the GSY1 promoter, which includes two STREs and three distinct negative elements, was surprising. This complexity may allow GSY1 to respond to a wide range of environmental stresses.The yeast Saccharomyces cerevisiae is exposed to a wide variety of environmental stressors, such as growth into stationary phase, heat shock, and osmotic shock. Cells respond to these stressors by modifying many metabolic processes, presumably making cells more resistant to these stressors. Among these changes are increased activities of the glycogen metabolic enzymes and the deposition of glycogen up to 23% of the dry weight of the cell (see below).Glycogen metabolism is highly conserved from yeast to mammals. The regulation of glycogen metabolism in S. cerevisiae closely parallels the more extensively studied counterparts in mammals (1, 2). Regulation is mediated primarily by effects on the activities of glycogen synthase and glycogen phosphorylase. These enzymes are regulated, in part, by protein phosphorylation. Cyclic AMP appears to play a central role in the regulation of these enzymes in S. cerevisiae, as it does in mammals, although the precise mechanisms remain to be identified. The parallels between glycogen metabolism in S. cerevisiae and mammals extends to the level of protein sequence. Glycogen synthase and glycogen phosphorylase from this yeast and mammals are 50 and 49% identical, respectively (3-6).Glycogen metabolism is also regulated at the level of gene expression in S. cerevisiae. The protein levels of the enzymes involved in glycogen metabolism increase in parallel with glycogen accumulation as cells approach stationary phase or when nutrients are depleted (7,8). This increase in the level of glycogen metabolic enzyme activity appears to result, in part, from the regulation at the level of transcription. Northern blot analysis has shown that the levels of mRNA expressed from GPH1 (encoding glycogen phosphorylase), GLC3 (glycogen branching enzyme)...