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When niacin-requiring E. coli was starved of niacin, a precursor of nicotinamide adenine dinucleotide (NAD), the cells elongated and synthesis of DNA was inhibited while syntheses of RNA and protein continued. The cellular NAD concentration decreased to less than one tenth of the normal level in niacin deficiency, but no change in DNA ligase level was detected. The apparent inhibition of DNA synthesis can be explained as due to reduction in the activity of the cofactor, NAD, for the enzyme. The DNA synthesized in this state was small in an alkaline sucrose gradient but of normal size in a neutral sucrose gradient. The molecules had a double-stranded structure, as judged by hydroxyapatite chromatography. The small pieces of DNA were integrated into normal-size DNA molecules on addition of niacin, when the cellular NAD concentration was restored to the normal level but DNA synthesis was not. It is suggested that this small DNA is an intermediate in DNA replication.

REPLICATION AND PROPERTIES OF DNA IN NICOTINAMIDE ADENINE DINUCLEOTIDE DEFICIENCY OF Escherichia coli CELLS