Multiplication of a restriction-modification gene complex

Previous works have suggested that some gene complexes encoding a restriction (R) enzyme and a cognate modification (M) enzyme may behave as selfish mobile genetic elements. RM gene complexes, which destroy 'non-self' elements marked by the absence of proper methylation, are often associated with mobile genetic elements and are involved in various genome rearrangements. Here, we found amplification of a restriction-modification gene complex. Bam HI gene complex inserted into the Bacillus chromosome showed resistance to replacement by a homologous stretch of DNA. Some cells became transformed with the donor without losing Bam HI. In most of these transformants, multiple copies of Bam HI and the donor allele were arranged as tandem repeats. When a clone carrying one copy of each allele was propagated, extensive amplification of Bam HI and the donor unit was observed in a manner dependent on restriction enzyme gene. This suggests that restriction cutting of the genome participates in the am-plification. Visualization by fluorescent in situ hybridization revealed that the amplification occurred in single cells in a burst-like fashion that is reminiscent of induction of provirus replication. The multiplication ability in a bacterium with natural capacity for DNA release, uptake and transformation will be discussed in relation to spreading of RM gene -complexes.