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The Essential Nature of the Ubiquitous 26-Kilobase Circular Replicon of Borrelia burgdorferi. Rebecca Byram, 2004.The genome of the type strain (B31) of Borrelia burgdorferi, the causative agent of Lyme disease, is composed of 12 linear and 9 circular plasmids and a linear chromosome . Plasmid content can vary among strains, but one 26-kb circular plasmid (cp26) is always present . The ubiquitous nature of cp26 suggests that it provides functions required for bacterial viability . We tested this hypothesis by attempting to selectively displace cp26 with an incompatible but replication-proficient vector, pBSV26 . While pBSV26 transformants contained this incompatible vector, the vector coexisted with cp26, which is consistent with the hypothesis that cp26 carries essential genes . Several cp26 genes with ascribed or predicted functions may be essential . These include the BBB29 gene, which has sequence homology to a gene encoding a glucose-specific phosphotransferase system component, and the resT gene, which encodes a telomere resolvase involved in resolution of the replicated telomeres of the linear chromosome and plasmids . The BBB29 gene was successfully inactivated by allelic exchange, but attempted inactivation of resT resulted in merodiploid transformants, suggesting that resT is required for B . burgdorferi growth . To determine if resT is the only cp26 gene essential for growth, we introduced resT into B . burgdorferi on pBSV26 . This did not result in displacement of cp26, suggesting that additional cp26 genes encode vital functions . We concluded that B . burgdorferi plasmid cp26 encodes functions critical for survival and thus shares some features with the chromosome .
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