We have investigated the mechanisms underlying resistance to the drug diazaborine in Saccharomyces cerevisiae. We used UV mutagenesis to generate resistant mutants, which were divided into three ...different complementation groups. The resistant phenotype in these groups was found to be caused by allelic forms of the genes AFG2, PDR1, and PDR3. The AFG2 gene encodes an AAA (ATPases associated to a variety of cellular activities) protein of unknown function, while PDR1 and PDR3 encode two transcriptional regulatory proteins involved in pleiotropic drug resistance development. The isolated PDR1-12 and PDR3-33 alleles carry mutations that lead to a L1044Q and a Y276H exchange, respectively. In addition, we report that overexpression of Yap1p, the yeast homologue of the transcription factor AP1, results in a diazaborine-resistant phenotype. The YAP1-mediated diazaborine resistance is dependent on the presence of functional PDR1 and PDR3 genes, although PDR3 had a more pronounced effect. These results provide the first evidence for a functional link between the Yap1p-dependent stress response pathway and Pdr1p/Pdr3p-dependent development of pleiotropic drug resistance.
The AAA protein Drg1 from yeast was affinity-purified, and its ATPase activity and hexamerization properties were analyzed. The same parameters were also determined for several mutant proteins and ...compared in light of the growth characteristics of the corresponding cells. The protein from a thermosensitive mutant exhibited reduced ATPase activity and hexamerization. These defects were not reversed by an intragenic suppressor mutation, although this allele supported growth at the nonpermissive temperature. A different set of mutants was generated by site-specific mutagenesis intended to adjust the Walker A box of the D2 domain of Drg1p to that of the D1 domain. A S562G exchange in D2 produced a nonfunctional protein that did not hexamerize but showed above-normal ATPase activity. The C561T mutant protein, on the other hand, was functional but hexamerized less readily and had reduced ATPase activity. In contrast, the C561T/S562G protein hexamerized less than wild type but had much higher ATPase activity. We distinguished strong and weak ATP-binding sites in the wild type protein but two weak sites in the C561T/S562G protein, indicating that the stronger site resides in D2. These observations are discussed in terms of the inter-relationship of ATPase activity per se, oligomeric status, and intracellular function for AAA proteins.
The EnvM protein was purified from an overproducing Escherichia coli strain. It showed NADH-dependent enoyl-acyl carrier protein
(ACP) reductase activity using both crotonyl-ACP and crotonyl-CoA as ...substrates. The protein bound a radioactive diazaborine
derivative in the presence of NAD+ and radioactive NAD+ in the presence of the drug. Based on these data, it is concluded
that EnvM is the NADH-dependent enoyl-ACP reductase (EC 1.3.1.9) of E. coli and we propose to rename the corresponding gene
fabI.
Upon Northern blotting, Saccharomyces cerevisiae that was treated with diazaborine showed aberrant mRNAs that were extended at the 3′-end and terminated at secondary processing sites. These bands ...were also detected in untreated Δupf1, Δxrn1, and rat7-1mutants. This finding demonstrates that the aberrant mRNAs also occur in untreated strains in small quantities and can reach the cytoplasm, where they are normally degraded by Xrn1p. Diazaborine treatment stabilizes these mRNAs. The detection of the aberrant bands in the untreated rat7-1 strain indicates that Rat7 is involved in quality control of RNA. The aberrant mRNAs were not detected after diazaborine treatment of a DRG1-1mutant. Drg1p, a member of the family of AAA (ATPases associated with a variety of cellular activities) proteins, which are thought to represent specific chaperones, may be involved in the process of unfolding the mRNA-ribonucleoprotein complex or in the recognition of aberrant mRNA molecules in the cytoplasm.
The gene (ERG1) encoding squalene epoxidase (ERG) from Saccharomyces cerevisiae was cloned. It was isolated from a gene library, prepared from an allylamine-resistant (Al(R)) S. cerevisiae mutant, by ...screening transformants in a sensitive strain for Al(R) colonies. The ERG tested in a cell-free extract from one of these transformants proved to be resistant to the Al derivative, terbinafine. From this result, we concluded that the recombinant plasmid in the transformant carried an allelic form of the ERG1 gene. The nucleotide sequence showed the presence of one open reading frame coding for a 55 190-Da peptide of 496 amino acids. Southern hybridization experiments allowed us to localize the ERG1 gene on yeast chromosome 15.
The role of the two-component response regulator ArcA protein in the transfer of the conjugative resistance plasmid R1 was investigated using a variety of
in vivo and
in vitro assays. The frequency ...of conjugal DNA transfer of plasmid R1-16, a derepressed variant of R1, was reduced by four orders of magnitude in an
Escherichia coli host with a mutation in the
arcA gene. Measurements of mRNAs transcribed from key plasmid transfer genes revealed that the abundance of each of the mRNA species investigated was reduced significantly in an
arcA background. Gene fusion studies with the R1 P
Y promoter, the major promoter of the transfer operon, and a
lacZ reporter gene, indicated that
arcA is required for maximal expression from this promoter. However, a stimulating effect of
arcA could only be detected when the plasmid-specified positive regulator of the transfer genes,
traJ, was present. Electrophoretic mobility shift assays were used to demonstrate specific binding of purified ArcA protein and a purified and phosphorylated oligohistidine-tagged ArcA (His6-ArcA) to a DNA fragment containing the P
Y promoter region. The binding of phosphorylated His6-ArcA to the P
Y promoter was further characterized by DNase I footprinting. The observed protection pattern was characteristic for ArcA acting as a transcriptional activator.
A key determinant of the frequency of IncF plasmid-mediated DNA transfer between enterobacterial cells is the FinOP system. traJ, a positive regulator of the transfer (tra) genes is controlled at the ...post-transcriptional level by two negative elements, finP and finO. FinP is a plasmid-specific antisense RNA, whereas finO encodes a proteic co-repressor which is not plasmid specific but exchangeable among F-like plasmids. We designed a traJ-lacZ test system that allowed us to monitor the effects of FinP and various FinP mutants on traJ expression. Furthermore, the introduction of finO into the test system enabled us to assess the function of FinO in the interaction of FinP with its target, the traJ mRNA. In this test system, FinP, expressed from a single-copy plasmid, in the absence of FinO, repressed traJ expression six-fold. When expressed from a pBR322-derived multicopy plasmid FinP repressed traJ expression approx. 2000-fold. This result unambiguously demonstrated that FinP is sufficient to repress traJ expression in a gene dosage-dependent manner. Mutations of finP creating base exchanges either in loop I or loop II of the two stem-loop structures of the antisense RNA led to a dramatic decrease in the repressor activity. In a combined loop I-loop II mutation the repressor activity was almost completely lost, supporting the model that the first critical interaction between the two RNA molecules occurs via 'kissing' of both loops of the RNAs. Addition of finO to the test system enhanced the repression of traJ expression by FinP by up to two orders of magnitude. This effect of FinO on FinP activity in vivo might indicate that FinO, in addition to its function as an RNA stabilizer, promotes complex formation between the target mRNA and the antisense RNA. Such a function of FinO has recently been shown to exist in vitro (van Biesen and Frost (1994) Mol Microbiol 14: 427-436).
Factors contributing directly to the cleavage of the conjugative transfer origin of plasmid R1 in Escherichia coli were investigated. The essential transfer protein TraM was identified as a necessary ...positive effector of the catalytic activity of TraI relaxase at the R1 transfer origin in the absence of protein TraY. The stimulatory effect of TraM on the cleavage reaction in vivo correlated with the capacity of TraM to bind origin DNA. TraM was shown to be essential for heterologous mobilization of recombinant origin DNA. The requirement for TraM to promote mobilization was distinct from the protein's positive effect on transfer gene regulation. Chimeric traM alleles, fusing heterologous amino and carboxyl coding sequences from the traM genes of the R1 and the IncFI plasmid P307, were used to localize the specificity determinant of TraM's DNA binding activity. Use of the chimeric alleles also revealed that the requirement for TraM in mobilization is origin specific but transfer system independent. No evidence was found for a plasmid specific activity of TraM at a stage in the transfer process subsequent to the initial cleavage of origin DNA. In light of TraM's regulatory functions in transfer gene expression, we propose that TraM could control conjugative DNA processing in response to intracellular levels of transfer proteins.
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