Fatty Acid Degradation in Escherichia coli: An Inducible Acyl‐CoA Synthetase, the Mapping of old‐Mutations, and the Isolation of Regulatory Mutants

Abstract

An enzymatic activity is described in oleate‐grown cells of E. coli, which has the cofactor requirements of a long chain acid: CoA ligase (AMP). This enzyme is at least partially membrane bound and activates saturated and mono‐ and polyunsaturated fatty acids. It is inactive with butyrate and hexanoate. The enzyme is induced by oleate under similar conditions, but at different relative rates, as three other enzymes of fatty acid degradation (thiolase, crotonase, hydroxyacyl‐CoA dehydrogenase) which are under coordinate control. A mutant lacking acyl‐CoA synthetase synthesizes the other three enzymes at the basal rate of the wild type in the presence or absence of oleate in the medium. This suggests that the fatty acyl‐CoA derivatives rather than the fatty acids serve as inducers. The same mutant is unable to incorporate fatty acids into bacterial lipids. The acyl‐CoA synthetase is therefore required for lipid synthesis from exogenous fatty acids in vivo. The isolation and properties of mutants unable to degrade oleate are described. A pleiotropic mutant lacking thiolase, crotonase and hydroxyacyl‐CoA dehydrogenase still contains the inducible acyl‐CoA synthetase. Mutants lacking thiolase and hydroxyacyl‐CoA dehydrogenase have inducible wild type levels of the other enzymes. Mutants affecting the synthesis of thiolase, hydroxyacyl‐CoA dehydrogenase or crotonase map between the loci metE and rha on the E. coli‐chromosome and are probably closely linked. The mutant lacking acyl‐CoA synthetase is not linked to the other mutants and maps near the his‐locus. Only fatty acids with more than twelve carbon atoms serve as inducers or can be used as sole carbon source by the wild type. This specificity is therefore typical for the abolition of repression by the interaction with the fatty acids. Spontaneous mutants can be isolated which grow on decanoate. These are constitutive for the synthesis of all four enzymes investigated and can only grow on fatty acids with more than eight carbon atoms. Thus, after destruction of the repression mechanism the growth behavior is a reflection of the substrate specificity of the acyl‐CoA synthetase. Starting with the constitutive mutants, spontaneous mutants have been isolated which can grow on butyrate. These fall into two classes: (a) One class grows on butyrate and poorly on long chain fatty acids. It makes all four enzymes constitutively but has twenty times more thiolase activity than the parent. This thiolase activity is not due to a gene linked to metE. (b) The other class can grow on butyrate and fatty acids with more than eight carbon atoms. It synthesizes the enzymes constitutively. The data suggest that the expression of the linked structural genes of crotonase, hydroxy‐acyl‐CoA dehydrogenase and thiolase and the unlinked gene of the acyl‐CoA synthetase from a regulon which is controlled by a common regulator gene.