Regulation of an IMP Dehydrogenase Gene and Its Overexpression in Drug-sensitive Transcription Elongation Mutants of Yeast

Abstract

IMP dehydrogenase is a rate-limiting enzyme involved in the synthesis of GTP. In mammalian cells it is regulated with respect to growth rate and is the target of numerous therapeutic agents. Mutations in the RNA polymerase II elongation machinery render yeast sensitive to inhibitors of IMP dehydrogenase and defective in inducing transcription of one of the IMP dehydrogenase-encoding genes, IMD2. Here we show that loss of IMD2, but not IMD1, IMD3, or IMD4, conferred upon yeast the same drug sensitivity found in elongation mutants. We tested whether the drug sensitivity of elongation mutants is due to their inability to induce IMD2 by providing them with exogenous copies of the gene. In some elongation mutants, overexpression reversed drug sensitivity and a transcriptional defect. Overexpression in mutants with a more severe phenotype partially suppressed drug sensitivity but was inconsequential in reversing a defect in transcription. These findings suggest that the drug sensitivity of elongation mutants is largely but not solely attributable to defects in the ability to induce IMD2, because transcription is compromised even when IMD2 mRNA levels are adequate. We describe two DNA sequence elements in the promoter of the gene that regulate it. We also found that IMD2 mRNA abundance is coupled to cell growth rate. These findings show that yeast possess a conserved system that gauges nucleotide pools and cell growth rate and responds through a uniquely regulated member of the IMD gene family.

Accumulation of unstable promoter-associated transcripts upon loss of the nuclear exosome subunit Rrp6p in Saccharomyces cerevisiae

“…5C) [although inhibition of overall transcription is a consequence of lowering intracellular NTP levels (27), we do not yet know whether this RNA is more inhibited than the average RNA by 6AU]. In contrast, the IMD2 mRNA is readily induced in wild-type cells (31), and this does not require Rrp6p (Fig. 5C).…”

Regulation of Amino Acid, Nucleotide, and Phosphate Metabolism in Saccharomyces cerevisiae

“…In contrast, inhibitors of IMPDH activity, such as mycophenolic acid (MPA) or 6-azauracil (6AU) that lower GTP pools (Exinger and Lacroute 1992) 2003b; Kuehner and Brow 2008). A repressive element was also identified between the TATA box and the transcriptional start site (Shaw et al 2001) and was shown to have transcription terminator properties (Jenks et al 2008).…”

“…Interestingly, in Bacillus subtilis, diminution of intracellular GTP concentration is required for proper entry into stationary phase (Ratnayake-Lecamwasam et al 2001). In yeast, expression of IMPDH-coding genes (IMDs) is strongly affected by growth phase (Shaw et al 2001;Escobar-Henriques et al 2003a). A mutagenesis of the IMD2 gene revealed that the sequence involved in growth phase regulation lies in the coding region of the gene (Escobar-Henriques et al 2003a).…”

Heterologous expression of a plant RelA-SpoT homologue results in increased stress tolerance in Saccharomyces cerevisiae by accumulation of the bacterial alarmone ppGpp

“…The GTP level is closely linked to ppGpp synthesis because (1) ppGpp synthesis consumes GTP as a substrate, and (2) ppGpp potently blocks GTP synthesis by inhibiting IMP dehydrogenase activity. The first effect (consumption of (Shaw et al, 2001). As ppGpp did not inhibit the IMP dehydrogenase activity of S. cerevisiae (Table 2), eukaryotic IMP dehydrogenases may be insensitive to ppGpp.…”

“…In mammalian cells, IMP dehydrogenase is a rate-limiting enzyme involved in de novo GTP biosynthesis. Therefore, we examined whether ppGpp inhibits yeast IMP dehydrogenases, which are present as four isozymes (IMD 1-4 encoded by YAR073W, YHR216W, YLR432W and YML056C, respectively) (Shaw et al, 2001). IMD1 is an inactive pseudogene.…”