The homologous terminal sequence of the Streptomyces lividans chromosome and SLP2 plasmid.

The chromosome of Streptomyces lividans shares 15.4 kb homology with one end of the linear plasmid SLP2, consisting of a 10.1 kb terminal sequence followed by the 5.3 kb transposable element Tn4811. The 10.1 kb terminal sequence was determined. The mean G+C content of this sequence is 67.9 mol% with a striking G vs C bias in the last kb. The terminal 232 nt contained 10 palindromic sequences with potential to form complex secondary structures. One typical Streptomyces coding sequence (designated ORF1) of 2643 bp was predicted in the determined sequence. The amino acid sequence of the ORF1 product contained a DEAH helicase motif, and exhibited similarity to type I restriction enzyme HsdR subunits in the database, suggesting a possible role in replication of the telomeres. However, all the ORF1 sequences on the chromosome and SLP2 could be simultaneously knocked out by targeted recombination without affecting the viability of the cells and the linearity of the chromosome and SLP2. This ruled out ORF1 as an essential component in the maintenance of the linear chromosome and plasmids.