Radioactive diazouracil (DZU-2-14C) was rapidly incorporated into acid-insoluble material of Escherichia coli B. Isolated ribonucleic acid contained essentially all of the incorporated label, and this was solubilized by ribonucleases but not by deoxyribonuclease. A maximum of 45 to 50% of added label was incorporated at division-inhibitory and subinhibitory concentrations. Incorporation levels and filament-forming capacity were concomitantly depressed by preincubation of DZU in various medium components. The lower levels of incorporation brought about by preincubation were apparently related to an inherent instability or high reactivity of the DZU. The limit on incorporation of fresh DZU suggests the presence of geometrical isomers in the original DZU. The nature of these isomers and the reactivity of DZU are discussed in relation to its use in future cell division studies.2-'4C and nonradioactive DZU synthesized by this procedure had matching infrared spectra. Two possible forms of the product, a hydrate and an anhydride, are shown in Fig.
E. coli
B, filamented with 5-diazouracil (DZU)-
2
-
14
C
, yielded ribonucleic acid (RNA)-(DZU-
2
-
14
C
) which was converted by pancreatic ribonuclease to
14
C-mono-and oligo-nucleotides. The mixed
14
C-mononucleotides isolated by diethylaminoethyl-cellulose fractionation were identified as cytidylic, uridylic, and hydroxyuridylic acids, by using a combination of paper chromatography and treatment with alkaline phosphatase and cytidine deaminase. Rifampin blocked incorporation of DZU-
2
-
14
C
under conditions which inhibit RNA synthesis. Division inhibition by DZU-
2
-
14
C
and the incorporation into
Escherichia coli
B were retarded by uracil but not by other RNA bases. In a pyrimidine-requiring
E. coli
, DZU substituted for uracil or cytosine to an extent limited by toxic effects. Cytosine and uracil retarded these effects and retarded the incorporation of DZU-
2
-
14
C
into the pyrimidineless strain. A small proportion of DZU-
2
-
14
C
was converted by the latter strain into hydroxyuridylic acid, but the bulk of the incorporated label was in cytidylic and uridylic acid, as in the wild strain.
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