Occurrence of a Catabolic l-Serine (l-Threonine) Deaminase in Saccharomyces cerevisiae

Saccharomyces cerevisiae mutants lacking the anabolic l-threonine deaminase, the ilvl−mutants, have been found to exhibit a normal ability to grow, without auxotrophy towards isoleucine, on l-threonine or l-serine as only nitrogen nutrient. Starting from a strain carrying a ilvl−mutation, which also impairs the ability to utilize l-serine, has been denominated cha−, for ‘catabolism of hydroxyamino acids’ and was found to result in the lack of a catabolic l-serine (l-threonine) deaminase. This enzyme which, unlike the anabolic threonine deaminase, is more active towards serine that towards threonine, differs from the latter enzyme by a number of biochemical and regulatory properties. Whereas the anabolic enzyme is an allosteric enzyme sensitive to feedback inhibition by isoleucine, the catabolic enzyme exhibits Michaelian kinetics; no control of its activity has been detected. Its synthesis is induced by l-serine and l-threonine. These two enzymes, which thus can be easily differentiated by means of their regulations, display a limited ability to compensate for one another's absence and appear to play clearly distinct roles under normal physiological conditions.