A novel system of bacterial cell division arrest implicated in horizontal transmission of an integrative and conjugative element

Integrative and conjugative elements (ICEs) are widespread mobile DNA elements in the prokaryotic world. ICEs are usually retained within the bacterial chromosome, but can be excised and transferred from a donor to a new recipient cell, even of another species. Horizontal transmission of ICEclc, a prevalent ICE in proteobacteria, only occurs from developed specialized transfer competent (tc) cells in the donor population. tc cells become entirely dedicated to the ICE transmission at the cost of cell proliferation. The cell growth impairment is mediated by two ICEclc located genes, parA and shi, but the mechanistic and dynamic details of this process are unknown. To better understand the function of ParA and Shi, we followed their intracellular behavior from fluorescent protein fusions, and studied host cell division at single-cell level. Superresolution imaging revealed that ParA-mCherry colocalized with the host nucleoid while Shi-GFP was enriched at the membrane during the growth impairment. Despite being enriched at different cellular locations, the two proteins showed in vivo interactions, and mutations in the Walker A motif of ParA dislocalized both ParA and Shi. In addition, ParA mutations in the ATPase motif abolished the growth arrest on the host cell. Time-lapse microscopy revealed that ParA and Shi initially delay cell division, suggesting an extension of the S phase of cells, but eventually completely inhibit cell elongation. The parA-shi locus is highly conserved in other ICEclc-related elements, and expressing ParA-Shi from ICEclc in other proteobacterial species caused similar growth arrest, suggesting that the system functions similarly across hosts. The results of our study provide mechanistic insight into the novel and unique system on ICEs and help to understand such epistatic interaction between ICE genes and host physiology that entails efficient horizontal gene transfer. Author summary Horizontal gene transfer is a major driving force for bacterial evolution, which is frequently mediated by mobile DNA vectors, such as plasmids and bacteriophages. Integrative and conjugative elements (ICEs) are relatively newly discovered mobile vectors, which are integrated in a host chromosome but under certain conditions can be excised and transferred from the host to a new recipient cell via conjugation. Recent genomic studies estimated that ICEs are widespread among bacteria, raising the question of the factors promoting their wide prevalence. One of the characteristics of ICEclc, an ICE model in proteobacteria, is that it develops specialized cells, which become entirely dedicated to the ICE horizontal transmission by repressing their proliferation. Here, we qualitatively and quantitatively describe functions of two key ICE proteins, which control cell growth arrest in transfer-dedicated cells. The two proteins are widely conserved among other ICEs of the same family, suggesting that the system of growth arrest in transfer-dedicated cells has been co-selected as a means for efficient ICE horizontal transmission.