Nucleotide sequence of the Salmonella typhimurium origin of DNA replication

Construction of deletion derivative plasmids and cloning of restriction fragments from plasmids containing the Salmonella typhimurium origin of replication (ori) were used to locate the functional origin to within a DNA fragment of 296 base pairs between the genes uncB and asn. The nucleotide sequence of the S. typhimurium ori region was determined and compared with the Escherichia coli ori sequence. In the 296-base pair fragment, 85.8% of the bases are conserved between the two species. A nearly equal number of transition and transversion type differences, with no insertions or deletions, occurs between the two bacterial origins, such that the relatively high percentage (adenine plus thymine) of 59.5% is conserved. The 296-base pair fragment contains 14 GATC sequences, all of which are conserved. The high frequency of occurrence of GATC, which is the site of methylation under control of the dam gene, may explain in part why the bacterial ori region appears to be so highly conserved. A large number of secondary structures are possible. One such structure, with a "cloverleaf," is favored by ori nucleotide sequence comparisons and leads to potential novel macromolecular interactions.