In the processes involved in the termination of replication of the circular chromosome of Escherichia coli, it has been determined that the replication terminus is opposite the replication origin and is located somewhere between the trp and his loci, but the position of the replication terminus has not yet been reported with any greater precision (1-4). It is also not certain if the replication terminus is a definite locus on the chromosome or if it is simply wherever the two replication forks involved in bidirectional replication happen to meet. Various investigators have reported results that suggest that replication occurs in a unidirectional fashion in certain situations (5-7). This suggests that there might not be a locus between the trp and his loci that blocks replication forks. Termination has now been studied with several different chromosomes that replicate bidirectionally, and the results demonstrate that bacteriophage X (8) and simian virus 40 (9) do not possess a specific region of the chromosome that blocks replication forks. The plasmid R6K, however, does have a terminus that blocks replication forks (10). The Bacillus subtilis chromosome might also have a specific terminus (11-13).In order to facilitate the study of termination in E. coli, we have sought conditions in which the normal symmetrical pattern of replication of the chromosome was altered so that one replication fork would reach the terminus region earlier than the other. We have recently determined that induction of prophage P2sig5 causes bacterial chromosome replication from the site of insertion of the prophage, and one of the strains we have studied has the prophage integrated near the terminus region. The results of studies of this strain are reported here. MATERIALS AND METHODSBacterial Strains and Media. PK241 is an F-thr leu his arg thi thyA drm dnaA malA+ E. coli strain derived from CRT4624. This latter strain and bacteriophage P2tsD4csigj (which will be called P2sig5 in this paper) were obtained from Y. Hirota (14). PK289 is an F-ilv arg met thi his thyA drm E. coli strain. PM 14 is a met trp his thr thy nic ilv Proteus mirabilis strain. The PM14 F' merogenote strains containing the F'129, F'116, F'111, F'101, and F'152 episomes were obtained by mating PM14 with E. coli strains containing these episomes and selecting for His+, Thy+, Ilv+, Thr+, and Gal+ cells, respectively. The E. coli F' strains were obtained from B. Bachman.The sources of the A specialized transducing phage were as follows: Xgt-E. coli EcoRI lop-li lig+ 1 (Alig), from R. Davis (15); XflaN+ 36 (Xfla), from M. Simon; A reverse (Arev), from M. Gellert (16); XptrpED 1 (Xtrp), from C. Yanofsky (17); XCIam34bio256 (Xbio), from M. Furth. XaroD was obtained by using the procedure of Schrenk and Weisberg (18).Cells were usually grown in M9 medium (19). During incubation in the absence of required amino acids, arginine or methionine was not removed from the medium. When PK241-P2-1 cells were labeled with 32P, they were grown in low-phosphate medium (20) containing 0.2...

Escherichia coli CRT4624-P2sig5 is a dnaA mutant in which integration of the prophage P2sig5 has occurred at the attP2II site (min 85). This strain was integratively suppressed, and when cells were shifted to 420C replication was initiated at a site in or near the P2 prophage. Initially, this replication occurred primarily in the direction that corresponds to the clockwise direction on the genetic map. Replication also occurred in the counterclockwise direction, but the initiation of replication in this direction occurred approximately 40 min later than the initiation of replication in the other direction. Because of this delay, the replication forks that traveled in the clockwise direction were the first to arrive in the region of the replication terminus. These replication forks ceased replication near the aroD locus (min 37), and it is proposed that the replication terminus is between the aroD and rac loci (min 31). A model is proposed for the cycle of chromosome replication in this strain at 420C.

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