Pyrimidine metabolism in Lactobacilli

Abstract

Pyrimidine metabolism in Lactobacilli was investigated with emphasis on L. fermenti. Pyrimidine, including thymine (thymidine) auxotrophs of L. fermenti, were specially isolated for this study. It was found that L. fermenti was sensitive to all 5-fluoropyrimidine analogues. That the toxicity of 5-fluorodeoxyuridine was exerted only on thymidylate synthetase was supported by two observations: (1) A thymine (thymidine) auxotroph was sensitive to all 5-fluoropyrimidine analogues except 5-fluorodeoxyuridine; (2) using the wild type 5 methyldeoxycytidine relieved the inhibition by 5-fluorodecxyuridine but not that of 5-fluorodeoxycytidine which also indicates that 5-fluorodeoxycytidine is toxic to the cells by being converted to 5-fluorodeoxyuridine triphosphate. Pyrimidine requirements of the pyrimidine auxotroph were satisfied by cytosine, cytidine, uracil and uridine while thymine, thymidine, and 5-methyIdeoxycytidine satisfied the requirement for thymine (thymidine) auxotrophy. In the three species under study, each had the enzymes cytidine (deoxycytidine) deaminase, deoxycytidine triphosphate deaminase, thymidine phosphorylase, and uridine phosphorylase. However, L. fermenti alone, possesses cytosine deaminase activity while L. acidophilus and L. leichmannii are devoid of it. The aspartate transcarbamylase of L. fermenti is repressed by uracil and cytosine, while it was not possible to find any nucleoside or nucleotide that feedback inhibited the well known regulatory enzyme. The velocity-substrate plots for aspartate transcarbamylase were hyperbolic with respect to L-aspartate and Michaelis-Menten kinetics were found.