The Linear Chromosomes of Streptomyces: Structure and Dynamics.
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概要
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The linearity of the chromosome has been observed in several <I>Streptomyces</I> species, and thus may be characteristic of the genus. The termini of the chromosomes contain inverted repeats, and carry covalently-bound proteins at the 5' end, presumably the primer for replication. The terminal inverted repeats of the <I>Streptomyces lividans</I> chromosome are about 30 kb long, of which the extreme 16 kb is indistinguishable from that of the right end of the 50-kb linear plasmid SLP2. The terminal sequence of <I>S. lividans</I> exhibits abundant palindromes, resembling those of several linear <I>Streptomyces</I> plasmids for the first 12∼17 bp. In spite of this, the terminal DNA sequences among different species are not highly conserved over long distances.<BR>A functional <I>oriC</I> has been located at the center of the chromosomes of <I>Streptomyces coelicolor</I> and <I>S. lividans</I>. These chromosomes appear to replicate bi-directionally from the center and are presumably patched up at the ends by protein-primed replication. At least in <I>S. lividans</I>, the telomeres are dispensable and the chromosome could be circularized in viable cells by targeted recombination, which removed the telomeres and joined the two arms. The circularized chromosomes were unstable, undergoing further rearrangements, and resulted in relatively poor growth and sporulation.<BR>Long stretches of DNA on various <I>Streptomyces</I> chromosomes are prone to spontaneous deletions and amplifications. In <I>S. lividans</I> and <I>S. coelicolor</I>, the unstable regions correspond to the termini. Moreover, circularized chromosomes, containing deletions of several hundreds of kb in the terminal regions, were found in some spontaneous deletion mutants of <I>S. lividans</I> ZX7. These observations indicate the readiness of the linear chromosome to be circularized, and implicate the termini in the structural instability of <I>Streptomyces</I> chromosomes.
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