Abstract
During conjugation, a conjugative DNA element is transferred from a donor to a recipient cell via a connecting channel. Conjugation has clinical relevance because it is the major route for spreading antibiotic resistance and virulence genes. The conjugation process can be divided into different steps. The initial steps carried out in the donor cell culminate in the transfer of a single DNA strand (ssDNA) of the conjugative element into the recipient cell. However, stable settlement of the conjugative element in the new host requires at least two additional events: conversion of the transferred ssDNA into double-stranded DNA and inhibition of the hosts’ defence mechanisms to prevent degradation of the transferred DNA. The genes involved in this late step are historically referred to as establishment genes. The defence mechanisms of the host must be inactivated rapidly and—importantly—transiently, because prolonged inactivation would make the cell vulnerable to the attack of other foreign DNA, such as those of phages. Therefore, expression of the establishment genes in the recipient cell has to be rapid but transient. Here, we studied regulation of the establishment genes present on the four clades of the pLS20 family of conjugative plasmids harboured by different Bacillus species. Evidence is presented that two fundamentally different mechanisms regulate the establishment genes present on these plasmids. Identification of the regulatory sequences were critical in revealing the establishment regulons. Remarkably, whereas the conjugation genes involved in the early steps of the conjugation process are conserved and are located in a single large operon, the establishment genes are highly variable and organised in multiple operons. We propose that the mosaical distribution of establishment genes in multiple operons is directly related to the variability of defence genes encoded by the host bacterial chromosomes.
Highlights
Conjugation is a horizontal gene transfer (HGT) route by which a DNA element is transferred from a donor to a recipient cell via a connecting channel
Conjugative elements can be located in a bacterial chromosome or plasmid, and are named integrated conjugative elements (ICEs) or conjugative plasmid, respectively
Some establishment genes are expressed in the donor cell, and the synthesised proteins are transported into the recipient cell together with the DNA; most establishment genes are expressed only in the recipient cell
Summary
The REase will digest any foreign DNA entering a cell that is not properly methylated (including conjugative DNA) [1,2,3] This system can be considered an innate defence mechanism. Inactivation of the bacterial defence systems must occur temporarily, because prolonged inactivation would make the cell vulnerable to entry of other foreign DNA, e.g., phage DNA This implies that anti-restriction genes and other establishment genes must be regulated in a special way, such that they are expressed rapidly and transiently upon arrival of the plasmid in the recipient cell. We discuss the possibility that the diversity of the establishment genes is directly related to the defence genes encoded by the bacterial genome
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