Utilization of labeled exogenous deoxythymidine (TdR) for deoxyribonucleic acid (DNA) synthesis is widely used as a measure of DNA synthesis itself, on the assumption that the degree of participation (DP) of TdR in the overall synthesis of deoxythymidine monophosphate in DNA is constant under a variety of conditions. It is now found that in Escherichia coli DP (ratio of exogenous TdR incorporated into DNA to total TdR content of DNA) depends on many factors. In basal medium, DP is 38% in wild-type strain K-12SH and 48% in strain K-12SH28 mutated in the TdR phosphorylase gene. In thymine-requiring (T-) mutants having a functional TdR phosphorylase, DP is less than 100%. Shortening the doubling time of the cells by supplementing basal medium with all amino acids increases DP of K-12SH and K-12SH28 by 25%, and Tstrains do reach 100%. Slowing cell growth by lowering the temperature to 30 C results in a decrease of DP by 15%. In the logarithmic phase, DP is higher than in the beginning of the stationary phase. It appears that exogenous TdR is more convenient for DNA synthesis during faster cell growth.

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