Formation of Ribosylthymine in Escherichia coli

Ribosylthymine (identified by paper chromatography with authentic ribosylthymine in five different solvents) is formed in thymine-requiring mutants of Escherichia coli provided with thymine or thymidine. Ribosylthymine was not detected in DNA pulse-labeled with thymidine, and attempts to demonstrate phosphorylation of ribosylthymine in cell extracts were unsuccessful. Ribosylthymine in the culture medium (20 µg per ml) inhibits growth and thymine uptake, whereas deoxyribonucleosides facilitate thymine uptake. Thymidine is utilized by cells in preference to and more efficiently than thymine; however, thymidine in the medium is very rapidly broken down to thymine. Some properties of an host cell reactivation-deficient and thymine-requiring mutant of E. coli (HF 4704) relevant to its use in studies of DNA biosynthesis are reported; in particular, it belongs to a class of mutants that are thymine-requiring at 37°, but not at 30°.