Abstract
The linearity of the chromosome has been observed in several Streptomyces 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 Streptomyces lividans 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 S. lividans exhibits abundant palindromes, resembling those of several linear Streptomyces 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.
A functional oriC has been located at the center of the chromosomes of Streptomyces coelicolor and S. lividans. 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 S. lividans, 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.
Long stretches of DNA on various Streptomyces chromosomes are prone to spontaneous deletions and amplifications. In S. lividans and S. coelicolor, 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 S. lividans ZX7. These observations indicate the readiness of the linear chromosome to be circularized, and implicate the termini in the structural instability of Streptomyces chromosomes.
