Ali Karami
Research Center of Molecular Biology, Baqyiatallah
University of Medical Sciences, Tehran
Iran

Introduction:
Borrelia may be unique among prokaryote in having a genome that is mainly linear DNA physical and genetic map of linear chromosome of B. burgdurferi has been published, it consist of 946 to 952 kb Linear DNA (Sherwood et al;1993, Davidson et al;1992, Barbour et al; 1982).This bacteria also contains several circular and specially linear plasmids from 5 to 55 kb. Recently analysis of entire Agrobacterium tumefaciens C58 genome revealed presence of one 2.1-Mb linear and one 3- Mb circular plasmid (Servent et al; 1993) and it has been shown that rhodococcus fascians contains 4 Mb linear chromosome (Crespi et al; 1992). Presence of several linear plasmids seems the segmentation of Borrelias DNA to several linear pieces has led to the suggestion that the relatively small linear chromosome and the linear plasmids actually are minichromosoms. In B. hermsii it has been shown that total cellular DNA organized into several complete gnomes (Kitten et al; 1992) and it suggests that linear plasmids are like small chromosomes (Ferdows et al; 1989). Plasmid profile of B. burgdorferi from different geographical area has been revealed significant heterogeneity a feature that can be used for classification of bacteria within given species (Barbour et al; 1987, 1989). Another related spirochete B. hermsii like B. burgdorferi has several linear and circular plasmids and the genes responsible for antigenic variation are located in linear plasmids. In B. burgdorferi a 49 kb linear plasmid carries the genes for Outer Surface Protein A and B (OspA and OspB)(Barbour et al; 1987, Baril et al;1989). It has been shown that passage of B. burgdorferi in BSK medium changes the plasmid profile and loss of plasmids may change the infectivity of organism (Schwan et al; 1988, Simpson, et al; 1990). Structure of Linear plasmids of B.burgdorferi shows similarity to eukaryotic virus such as vaccinia and African swine fever virus in having covalently closed ends like hairpin loops (Hinnebusch et al;