Hisao Masai and Ken-ichi Arai.

Department of Molecular and Developmental Biology, Institute of Medical Science, University of Tokyo, 4-6-1, Shirokanedai, Minato-ku, Tokyo, 108, Japan

Received 01/17/96; Accepted 02/17/96; On-line 03/01/96

Replication of the E. coli chromosome under normal growth condition appears to be conducted solely by DnaA-dependent primosome and dispensability of PriA protein for viability indicates that the phiX174-type primosome is not involved in DnaA/oriC-dependent replication (3, 34). On the other hand, PriA-dependent SDR does not require dnaA function. Thus, DnaA-dependent and PriA-dependent primosomes function independently of each other. The E. coli cells can be replicated with either protein complex, depending on the environment or genetic background. Replication of various plasmids generally depends on host replication proteins except for plasmid-encoded initiators that specifically recognize cognate replication origins. Many replication origins contain one or more DnaA box sequences within the minimum replication origin sequences and their replication does require DnaA protein (38). Replication of these DnaA-dependent replicons, such as F, pSC101, R6K, RK2 or Rts1, does not require priA function, whereas replication of pBR322, RSF1030, and ColE2, which is independent of dnaA function, strictly requires PriA protein (26). These indicate that replication of plasmid replicons also can be classified into either DnaA-type or PriA-type (Table 2). In the former class, DnaA protein may help assembly of a replication complex similar to oriC-type or ABC primosome by binding to the DnaA box present within the replication origin, while, in the latter class, the phiX174-type primosome may be assembled at an n'-pas on the lagging strand template. We propose that E. coli cells possess two functionally similar, independent primosomes, which are differentially utilized by the chromosome as well as by various plasmids (Table 2).