Streptomyces genomes: circular genetic maps from the linear chromosomes

Streptomyces chromosomes are linear DNA molecules and yet their genetic maps based on linkage analysis are circular. The only other known examples of this phenomenon are in the bacteriophages T2 and T4, the linear genomic sequences of which are circularly permuted and terminally redundant, and in which replication intermediates include long concatemers. These structural and functional features are not found in Streptomyces. Instead, the circularity of Streptomyces genetic maps appears to be caused by a completely different mechanism postulated by Stahl & Steinberg (1964, Genetics 50, 531-538)--a strong bias toward even numbers of crossovers during recombination creates misleading genetic linkages between markers on the opposite arms of the chromosome. This was demonstrated by physical inspection of the telomeres in recombinant chromosomes after interspecies conjugation promoted by a linear or circular plasmid. The preference for even numbers of crossovers is probably demanded by the merozygosity of the recombining chromosomes, and by the association between the telomeres mediated by interactions of covalently bound terminal proteins.