H. E. Umbarger

Isoleucine and valine metabolism in Escherichia coli. XX. Multiple forms of acetohydroxy acid synthetase

Summary Acetohydroxy acid synthetase (AHAS) from Salmonella typhimurium strain TU5006 (derived from LT2) and Escherichia coli strain M-42-11 (derived from W) consists of two activities. One (AHAS I) is sensitive to feedback inhibition by isoleucine, leucine and valine, and the other (AHAS II) is insensitive to these same effectors. E. coli strain CU1013, a derivative of strain K-12, lacks the ability to produce AHAS II, thus accounting for the valine-sensitive growth characteristic of derivatives of E. coli strain K-12. Production of the AHAS I activity appears to be under multivalent control by valine and leucine, while that of AHAS II, in those strains that produce it, appears to be under multivalent control by isoleucine, valine and leucine.

The metabolism of valine and isoleucine in Escherichia coli XVII the role of induction in the depression of acetohydroxy acid isomeroreductase

Abstract Under conditions that repress the formation of four of the five enzymes required for isoleucine and valine biosynthesis in Escherichia coli and Salmonella typhimurium (growth in the presence of excess isoleucine, valine and leucine), the fifth enzyme, acetohydroxy acid isomeroreductase, is induced by its substrates, acetohydroxybutyrate and acetolactate. Substrate induction appears to be the major if not the only means for regulating the formation of the isomeroreductase in these organisms. The same process seems to occur in Aerobacter aerogenes .

Regulation of isoleucine-valine biosynthesis in an ilvDAC deletion strain of Escherichia coli K-12

Abstract Studies of isogenic strains of Escherichia coli K-12 with and without the ilvDAC115 deletion described by Kiritani showed that a strain carrying this lesion does not have the ilvA , ilvD or ilvC structural genes but has normal multivalent regulation of the ilvB and ilvE structural genes. It was also shown that the regulatory locus ( ilvO ) for the ilvADE operon (defined by Ramakrishnan and Adelberg) affects the expression of the ilvB and ilvC structural genes and is located outside of the ilvDAC115 deletion. These experiments demonstrate that there is multivalent control of at least two ilv biosynthetic enzymes in the absence of the ilvA gene product.

A role for a pyridoxine derivative in the multivalent repression of the isoleucine and valine biosynthetic enzymes

Abstract Starvation of a pdx mutant of Escherichia coli strain B in the presence of repressing levels of isoleucine, valine and leucine leads to a derepression of the normally repressible ilv genes. The derepression of the ilvA gene under these conditions results in the accumulation of apothreonine deaminase. Addition of pyridoxine leads to a sudden increase in threonine deaminase activity, and to restoration of repression. The pyridoxine component needed for the repression signal is probably not threonine deaminase but, more likely, some transient (“immature”) form of the enzyme.

SINGLE REACTIONS WITH MULTIPLE FUNCTIONS: MULTIPLE ENZYMES AS ONE OF THREE PATTERNS IN MICROORGANISMS

ABSTRACT: Diverging and converging pathways are frequently encountered in the study of cell metabolism and therefore a single chemical transformation is often found to serve multiple functions. Control of metabolite flow through multipurpose anabolic transformations can be achieved by concerted inhibition by the multiple endproducts, sequential inhibition by each branch point intermediate, or by uniquely controlled multiple enzymes (isozymes). Five distinct functional classes of the latter are discussed here. The catabolic and biosynthetic threonine deaminases and acetohydroxy acid synthases are considered in some detail as typical examples.