BACTERIAL PLASMIDS: REPLICATION OF EXTRACHROMOSOMAL GENETIC ELEMENTS ENCODING RESISTANCE TO ANTIMICROBIAL COMPOUNDS

Luis A. Actis,¹ Marcelo E. Tolmasky,² and Jorge H. Crosa³¹

Department of Microbiology, Miami University, Oxford, Ohio 45056-8900, ²Department of Biological Science, School of Natural Science and Mathematics, California State University Fullerton, Fullerton, California 92821-6850, ³Department of Molecular Microbiology and Immunology, School of Medicine, Oregon Health Sciences University, Portland, Oregon 97201-3098

TABLE OF CONTENTS

1. Abstract

2. Introduction

3. Replication

3.1. Iteron-regulated plasmids: the R6K plasmid
3.2. RNA-regulated plasmids: the ColE1-type plasmids
3.3. Rolling-circle replicating plasmids: the pT181-type plasmids
3.4. Replication of linear plasmids: the Streptomyces and Borrelia plasmids

4. Plasmid copy number and incompatibility

4.1. Antisense RNAs
4.2. DNA iterons

5. Acknowledgments

6. References

Plasmids are self-replicating extrachromosomal DNA molecules found in Gram-negative and Gram-positive bacteria as well as in some yeast and other fungi. Although most of them are covalently closed circular double-stranded DNA molecules, recently linear plasmids have been isolated from different bacteria. In general, plasmids are not essential for the survival of bacteria, but they may nevertheless encode a wide variety of genetic determinants, which permit their bacterial hosts to survive better in an adverse environment or to compete better with other microorganisms occupying the same ecological niche. The medical importance of plasmids that encode for antibiotic resistance, as well as specific virulence traits has been well documented and demonstrated the important role these bacterial genetic elements play in nature. Although they encode specific molecules required for initiation of their replication, plasmids rely on host-encoded factors for their replication. Plasmid replication initiates in a predetermined cis-site called ori and can proceed either by a rolling circle or a theta replication mechanism. Some of the plasmid-encoded elements required for their replication, such antisense RNA molecules and DNA repeated sequences located close to ori, determine plasmid attributes like copy number and incompatibility.