A new beginning with new ends: linearisation of circular chromosomes during bacterial evolution

Bacterial circular chromosomes have sporadically become linearised during p rokaryote evolution. Unrelated bacteria, including the spirochete Borrelia burgdorferi and the actinomycete Streptomyces, have linear chromosomes. Lin ear chromosomes may have been formed through integration of linear plasmids . Linear chromosomes use linear plasmid strategies to resolve the 'end-of-r eplication problem', but they have generally retained from their circular a ncestors a central origin of replication. Streptomyces linear chromosomes a re very unstable and at high frequency undergo amplifications and large del etions, often removing the telomeres. At least in Streptomyces, chromosome linearity is reversible: circular chromosomes arise spontaneously as produc ts of genetic instability or can be generated artificially by targeted reco mbination. Streptomyces circularised chromosomes are very unstable as well, indicating that genetic instability is not confined to the linearised chro mosomes. Bacterial linear chromosomes may contain telomere-linked regions o f enhanced genomic plasticity, which undergo more frequent genetic exchange s and rearrangements and allow differential evolution of genes, depending o n their chromosomal location. (C) 2000 Federation of European Microbiologic al Societies. Published by Elsevier Science B.V. All rights reserved.