The de novo pyrimidine biosynthetic enzymes in the denitrifying bacter
ium Pseudomonas stutzeri ATCC 17588 were assayed and their activities
were lower in glucose-grown cells than in succinate-grown cells. When
P. stutzeri was grown in the presence of uracil, the de novo enzyme ac
tivities in succinate-grown cells were lowered while they remained lar
gely unchanged in glucose-grown cells. A uracil auxotroph of P. stutze
ri, deficient for aspartate transcarbamoylase activity, was isolated a
nd its auxotrophic requirement was met by only uracil and cytosine. Th
e inability of pyrimidine ribonucleosides to meet the auxotrophic requ
irement was related to the limited ability of P. stutzeri to transport
uridine and cytidine. Pyrimidine limitation of the auxotroph elevated
the de novo enzyme activities indicating that this pathway may be rep
ressible by a uracil-related compound in succinate-grown P. stutzeri c
ells. Regulation of pyrimidine synthesis in P. stutzeri was similar to
that observed for other pseudomonads classified within rRNA homology
group I.