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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.

The broad-spectrum mercury resistance transposon, TnMERI1, of Bacillus megaterium strain MB1, contains three proposed operator/promoter (O/P) transcriptional start sites and two regulatory genes (merR1 and merR2). A series of luciferase (lux)-based transcriptional fusion plasmids were studied in Escherichia coli to show that both merR1 and merR2 gene products repressed transcription from O/PmerB3, O/PmerR1, and O/PmerR2 under uninduced conditions. Derepression occurred when the merR1 gene was present and Hg(2+) functioned as an inducer. In the presence of organomercurial compounds, basal transcription of merB3 was needed to produce inorganic Hg(2+) as the inducer of expression regulated by MerR1 at O/PmerB3. The presence of merR2 repressed transcription from all three O/Pmer sites under both non-induced conditions and when inorganic Hg(2+) or organomercurials were added. These results show that MerR1 functions as a repressor in the absence of Hg(2+) and as an activator in the presence of Hg(2+), while MerR2 functions as a repressor.