Efficiency of utilization of thymine and 5-bromouracil for normal and repair DNA synthesis in bacteria

Abstract

Thymine-requiring bacteria exhibit a pronounced selectivity for thymine over its analogue, 5-bromouracil, in normal DNA synthesis when both are supplied in the growth medium. The present report extends this observation to a consideration of the utilization of these pyrimidines in DNA repair replication. 1. 1. The selectivity for thymine over 5-bromouracil results from a more rapid reaction rate for utilization of thymine than of 5-bromouracil in one or more of the reactions by which exogenous pyrimidines are incorporated into DNA. 2. 2. A distinctly greater selectivity for thymine over 5-bromouracil is expressed in the non-conservative repair mode of DNA replication than in the semi-conservative mode. 3. 3. Both repair and normal DNA synthesis can be assumed to draw precursors from the same pool of nucleoside triphosphates. 4. 4. Our results support the hypothesis that separate enzyme systems are involved in the normal and in the repair modes of DNA replication and that the repair polymerase complex exhibits a more stringent requirement for the natural base, thymine. 5. 5. The loss in viability observed when bacteria grow in 5-bromouracil medium may be due to the relative inefficiency of utilization of 5-bromouracil in a required repair-replication step in normal DNA metabolism.