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In Escherichia coli mutants deficient in DNA polymerase I, newly -replicated short DNA is joined at about 10% of the rate in the wild-type strains. It is postulated that DNA polymerase I normally functions in filling gaps between the nascent short segments synthesized by the replication complex. Possible implications of the finding are discussed in relation to other abnormal properties of these mutants.Mutants of Escherichia coli deficient in DNA polymerase I (pob mutants) (1-3) grow normally. These mutants, however, exhibit ,a number of abnormal properties, such as high sensitivity to ultraviolet light, x-ray, and other mutagenic agents (1-8), incompatibility with a recA mutation (9), high frequency of chromosomal deletions (10), and inability to maintain a certain plasmid (11).We reported (12)

Evidence has accumulated that DNA is replicated by a discontinuous mechanism involving systhesis and joining of short DNA units (1-8). Little is known, however, about the mechanism of creation of the short DNA chains.We postulated (2) that short DNA units might be synthesized by a mechanism in which specific structures on the template strands serve as initiation or termination signals. Although short DNA similar to that found in vivo can be produced and joined in crude in vitro systems (refs. 9-13, Ogawa and Okazaki, to be published), synthesis of such DNA has not been demonstrated with purified DNA polymerases. Unlike RNA polymerases, the known DNA polymerases do not seem to initiate new chains along parental templates, but rather catalyze the extention of a preexisting chain (14-18). Furthermore, DNA polymerases have relatively poor template specificities (18,19), in contrast to those of RNA polymerases and other proteins involved in transcription (20,21). These facts, as well as the recent findings of Kornberg and coworkers (22, 23) indicating a requirement for RNA synthesis to produce a primer for initiation of replication of phage M13 DNA, suggest that RNA synthesis (transcription) might be involved in the initiation of synthesis of the short DNA chains at specific sites on the template in the discontinuous mode of replication. In accord with this possibility, we have obtained evidence that a short stretch of RNA is covalently linked to the nascent DNA fragments. MATERIALS AND METHODSThe following Escherichia coli strains were used: AB301 (Hfr met-X+); D10

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