Escherichia coli mutants with altered control of alcohol dehydrogenase and nitrate reductase.

Mutants of Escherichia coli which overproduce alcohol dehydrogenase were obtained by selection for the ability to use ethanol as an acetate source in a strain auxotrophic for acetate. A mutant having a 20-fold overproduction of alcohol dehydrogenase was able to use ethanol only to fulfill its acetate requirement, whereas two mutants with a 60-fold overproduction were able to use ethanol as a sole carbon source. The latter two mutants produced only 25% of the wild-type level of nitrate reductase, when grown under anaerobic conditions. Alcohol dehydrogenase production was largely unaffected by catabolite repression but was repressed by nitrate under both aerobic and anaerobic conditions. The genetic locus responsible for alcohol dehydrogenase overproduction was located at min 27 on the E. coli genetic map; the gene order, as determined by transduction, was trp tonB adh chlC hemA. The possible relationship of alcohol dehydrogenase to anaerobic redox systems such as formate-nitrate reductase is discussed.     

Acetaldehyde coenzyme A dehydrogenase of Escherichia coli.

Mutants of Escherichia coli (adh) in which alcohol dehydrogenase is derepressed under aerobic conditions were also found to overproduce acetaldehyde coenzyme a dehydrogenase. However, acetaldehyde coenzyme A dehydrogenase was induced by ethanol or acetaldehyde and subject to strong catabolite repression, whereas alcohol dehydrogenase was little affected by these conditions. Mutants no longer able to use ethanol as carbon source were isolated from an adh strain. Some of these mutants were revertants at the adh locus and no longer produced either alcohol dehydrogenase or acetaldehyde coenzyme A dehydrogenase. Others, designated acd, were found to lack only acetaldehyde coenzyme A dehydrogenase. The acd mutation was located at min 62 of the E. coli genetic map, the gene order being thyA-lysA-acd-serA-fda. Isolation of Tn10 insertions cotransducible with acd greatly simplified the mapping procedure.     

Induction of lactose transport in Escherichia coli during the absence of phospholipid synthesis.

Induction of lactose transport and of beta-galactosidase synthesis was examined in two Escherichia coli strains that require exogenous glycerol for phospholipid synthesis and growth. No preferential inhibition of lactose transport induction was observed when phospholipid synthesis was restricted to 5 to 10% of the normal rate. We conclude that the lactose transport system does not require concurrent phospholipid synthesis for its functional assembly.     

Mutant of Escherichia coli Deficient in the Synthesis of cis-Vaccenic Acid

A temperature-sensitive unsaturated fatty acid (fabA) auxotroph of Escherichia coli was found also to be deficient in the elongation of palmitoleic acid to cis-vaccenic acid. Reversion and transductional analyses demonstrate that this second phenotype and the fabA mutation are independent in action and are not cotransduced. The deficiency in conversion of palmitoleic acid to cis-vaccenic acid was also demonstrated in vitro, and these results strongly suggest this phenotype is due to a deficiency in an elongation enzyme. We suggest that the phenotype may have been selected during growth because it can physiologically compensate for the fabA lesion. In fab(+) strains, the inability to synthesize cis-vaccenic acid is physiologically asymptomatic. Such strains grow normally at all temperatures tested and are not sodium sensitive. Although the parental strain has an increased amount of cis-vaccenic acid in cells grown at 15 C, the mutant does not. Since the mutant grows normally at 15 C, the data indicate that increased amounts of cis-vaccenic acid are not required for growth at 15 C.     

Mechanism of phospholipid biosynthesis in Escherichia coli: Acyl-CoA synthetase is not required for the incorporation of intracellular free fatty acids into phospholipid

Previous results have shown that acyl-CoA synthetase is required both for the incorporation of exogenous fatty acids into the phospholipids of $\text{E. coli}$ and for the transport of fatty acids into the cell. We have demonstrated that acyl-CoA synthetase is not required for the incorporation of intracellular free fatty acids into phospholipid. This finding indicates that the role of this enzyme in the incorporation of exogenously supplied fatty acids is primarily at the level of fatty acid transport.     

Selection for the transfer of phenotypically nonselectable chromosomal mutations to recombinant plasmids through introduction of an altered restriction site

We describe a simple method to select for transfer of mutant alleles from the Escherichia coli chromosome to a plasmid which formerly carried the wild-type (wt) allele. The wt allele on the plasmid is modified by introduction of a unique restriction site (e.g., XhoI) and transformed into a rec ⁺ strain carrying the mutant allele on the chromosome. Upon homogenotization, the efficiency of which was increased by UV irradiation of the transforming plasmid [Chattoraj et al., Gene 27 (1982) 213–222], plasmids carrying the mutant allele are formed which are resistant to XhoI. These plasmids are selected from the population by resistance to XhoI digestion coupled with the low transformation efficiency of linear DNA molecules in recA⁻ strain. The method is efficient and rapid and has particular advantages in situations where the mutant allele is difficult to detect by its phenotype.     

Integrative analysis of multiple gene expression profiles with quality-adjusted effect size models

Background  

With the explosion of microarray studies, an enormous amount of data is being produced. Systematic integration of gene expression data from different sources increases statistical power of detecting differentially expressed genes and allows assessment of heterogeneity. The challenge, however, is in designing and implementing efficient analytic methodologies for combination of data generated by different research groups.     

The reaction of LipB, the octanoyl-[acyl carrier protein]:protein N-octanoyltransferase of lipoic acid synthesis, proceeds through an acyl-enzyme intermediate

The lipB gene of Escherichia coli encodes an enzyme (LipB) that transfers the octanoyl moiety of octanoyl-acyl carrier protein (octanoyl-ACP) to the lipoyl domains of the 2-oxo acid dehydrogenases and the H subunit of glycine cleavage enzyme. We report that the LipB reaction proceeds through an acyl-enzyme intermediate in which the octanoyl moiety forms a thioester bond with the thiol of residue C169. The intermediate was catalytically competent in that the octanoyl group of the purified octanoylated LipB was transferred either to an 87-residue lipoyl domain derived from E. coli pyruvate dehydrogenase or to ACP (in the reversal of the physiological reaction). The octanoylated LipB linkage was cleaved by thiol reagents and by neutral hydroxylamine, strongly suggesting a thioester bond. Separation and mass spectral analyses of the peptides of the unmodified and octanoylated proteins showed that each of the assigned peptides of the two proteins had identical masses, indicating that none of these peptides were octanoylated. However, the one major peptide that we failed to recover was that predicted to contain all three LipB cysteine residues. These three cysteine residues were therefore targeted for site-directed mutagenesis and only C169 was found to be essential for LipB function in vivo. The C169S protein had no detectable activity whereas the C169A protein retained trace activity. Surprisingly, both proteins lacking C169 formed an octanoyl-LipB species, although neither was catalytically competent. The octanoyl-LipB species formed by the C169S protein was resistant to neutral hydroxylamine treatment, consistent with formation of an ester linkage to the serine hydroxyl group. The octanoyl-C169A LipB species was probably acylated at C147. LipB species that lacked all three cysteine residues also formed a catalytically incompetent octanoyl adduct, indicating the presence of a reactive side chain other than a cysteine thiol that lies adjacent to the active site.