Abstract
It is well known that aspartate-carbamoyltransferase (ACTase) in E. coli is regulated by pyrimidine nucleotides. Since aspartate has some stimulatory effect on early thiamine synthesis in a thiazole-less mutant of E. coli and ^<14>C-aspartate is incorporated into the pyrimidine moiety of thiamine, the regulation of ACTase by thiamine or thiamine diphosphate was investigated by using a hydroxymethylpyrimidine-less mutant of E. coli. Although uracil-depletion in the mutant caused the elevation of ACTase activity about 25 fold compared with that of uracil-supplemented, hydroxymethylpyrimidine-depletion showed no effect on the activity and also thiamine or thiamine diphosphate added to the enzyme reaction mixture gave no appreciable effect on the activity. Judging from the point of physiological requirements of thiamine and pyrimidine nucleotides, it is difficult to know the relationship between ACTase and thiamine synthesizing system in E. coli.
