REPLICATION AND CONTROL OF CIRCULAR BACTERIAL PLASMIDS

An essential feature of bacterial plasmids is their ability to replica te as autonomous genetic elements in a controlled way within the host. Therefore, they can be used to explore the mechanisms involved in DNA replication and to analyze the different strategies that couple DNA r eplication to other critical events in the cell cycle. In this review, we focus on replication and its control in circular plasmids. Plasmid replication can be conveniently divided into three stages: initiation , elongation, and termination. The inability of DNA polymerases to ini tiate de novo replication makes necessary the independent generation o f a primer. This is solved in circular plasmids, by two main strategie s: (i) opening of the strands followed by RNA priming (theta and stran d displacement replication) or (ii) cleavage of one of the DNA strands to generate a 3'-OH end (rolling-circle replication). Initiation is c atalyzed most frequently by one or a few plasmid-encoded initiation pr oteins that recognize plasmid-specific DNA sequences and determine the point from which replication starts (the origin of replication). In s ome cases, these proteins also participate directly in the generation of the primer. These initiators can also play the role of pilot protei ns that guide the assembly of the host replisome at the plasmid origin . Elongation of plasmid replication is carried out basically by DNA po lymerase III holoenzyme land, in some cases, by DNA polymerase I at an early stage), with the participation of other host proteins that form the replisome. Termination of replication has specific requirements a nd implications for reinitiation, studies of which have started. The i nitiation stage plays an additional tale: it is the stage at which mec hanisms controlling replication operate. The objective of this control is to maintain a fixed concentration of plasmid molecules in a growin g bacterial population (duplication of the plasmid pool paced with dup lication of the bacterial population). The molecules involved directly in this control can be (i) RNA (antisense RNA), (ii) DNA sequences (i terons), or (iii) antisense RNA and proteins acting in concert. The co ntrol elements maintain an average frequency of one plasmid replicatio n per plasmid copy per cell cycle and can ''sense'' and correct deviat ions from this average. Most of the current knowledge on plasmid repli cation and its control is based on the results of analyses performed w ith pure cultures under steady-state growth conditions. This knowledge sets important parameters needed to understand the maintenance of the se genetic elements in mixed populations and under environmental condi tions.