A 2019 bp DNA fragment containing the replicon of pSW800 from Pantoea stewartii SW2 was cloned and characterized. This replicon contains two genes – repA and repB, which encode a 36·5 kDa replication initiation protein (RepA) and a peptide of 18 aa, respectively. These two genes overlap by 8 bases with repB situated upstream. The replicon also transcribes an antisense RNA (RNAI) that inhibits the expression of repA and repB. The ribosome-binding sequence (RBS) of repA is likely to be hidden in a stem–loop structure, inhibiting the translation of repA. Furthermore, translation of repB is likely to disrupt the stem–loop structure, which is one of the criteria allowing the translation of repA to begin. A mutagenesis study revealed that a sequence (5′-GCACGGG-3′) located 111 nt upstream from repA is crucial; mutation of this sequence prevented the translation of repA. Additionally, this region and the stem–loop structure containing the RBS of repA may form an RNA pseudoknot. Results in this study demonstrate that a mechanism similar to that regulating plasmid replication in the IncB, IncIα and IncL/M groups also regulates pSW800 replication.
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