Facilities for transcription and mobilization of an exon-less bacterial group II intron nested in transposon TnMERI1

Transcription and mobilization facilities of a group II intron, B.me.I1, which was classified within a group IIB subclass and nested in a broad-spectrum mercury resistance transposon TnMERI1 found from the chromosome of Bacillus megaterium strain MB1 were investigated. Though B.me.I1 does not intervene in any recognizable exon gene, the splicing ability of B.me.I1 in Escherichia coli was confirmed by applying RT-PCR with RNA template transcribed by using a T7 RNA polymerase-promoter expression system. The homing activity of B.me.I1 was confirmed by using an intron-less allele fragment as a specific integration site which was cloned from Tn5084, a TnMERI1-family mercury resistance transposon. The transcription and splicing of B.me.I1 in the original host, the strain MB1, were also observed as its natural property. Primer-walking and 5′ RACE analyses showed that the transcription start site and the putative promoter region of B.me.I1 were located on the antisense strand of tnpR gene of TnMERI1. Therefore, it is cleared that TnMERI1 provides a specific integration site and facilities for transcription of B.me.I1. From these results, it is considered that these genetic facilities given by the transposon TnMERI1 confer splicing and homing capability on B.me.I1, even though the group II intron is not associated with any exon gene.