Amino acid metabolism in Acetobacter suboxydans

Graduation date: 1964Nutritional studies with Acetobacter suboxydans indicated that\ud when isoleucine, lysine, methionine, serine and arginine were singly\ud omitted from a complete amino acid mixture, no growth was obtained.\ud However, the organism could grow on single amino acids like glutamic\ud acid, histidine and proline. No growth was obtained with ammonium\ud sulfate as the sole nitrogen source. An investigation on the cause\ud for the apparent requirement for isoleucine was made. When valine was\ud added to a synthetic medium containing histidine, glutamic acid, proline\ud and ammonium sulfate no growth was obtained. Valine appeared to inhibit\ud growth, an effect which could be reversed by addition of isoleucine.\ud Cell-free extracts of the organism synthesize both valine and isoleucine\ud from acetolactate and acetohydroxybutyrate respectively. The amounts\ud of the two amino acids synthesized from different intermediates were\ud determined.\ud A study of the mechanism of growth inhibition due to valine was\ud initiated. U-C¹⁴ threonine was incorporated into isoleucine by growing\ud cells. The first step in the biosynthesis of isoleucine from threonine is the deamination of the latter to ?-ketobutyrate. This enzyme,\ud threonine deaminase, was precipitated from cell-free extracts at 50\ud percent saturation with ammonium sulfate. Using this fraction, it was\ud found that the activity of the deaminase was competitively inhibited\ud by valine and isoleucine. Although isoleucine repressed the synthesis\ud of the deaminase, valine did not. Therefore one mechanism by which\ud valine inhibits growth of the organism is by feed-back inhibition of\ud the threonine deaminase thereby limiting isoleucine biosynthesis. This\ud is evident by an apparent requirement for isoleucine when valine is\ud present in the growth medium. No inhibition of the transport of isoleucine\ud across the cell wall was observed.\ud Mutants of Acetobacter suboxydans have been isolated which are\ud prototrophic towards isoleucine and resistant to growth inhibition by\ud valine. Some of these mutants possess a threonine deaminase which is\ud not sensitive to valine or isoleucine