Dynamics of bacterial cell proliferation inhibited by specific proteins for effective horizontal transmission of the integrative and conjugative element ICEclc

Abstract

The integrative and conjugative element ICEclc, a prevalent mobile element in proteobacteria, is one of the experimental models for horizontal gene transfer. ICEclc is usually retained in a bacterial chromosome, but can be excised and transferred from the donor to other bacterial lineages. The horizontal transmission is accomplished by developing specialized transfer competent (tc) cells in the donor population. The tc cells entirely dedicate to the ICE transmission by sacrificing their proliferation, which results in an increase in the transfer frequency. The cell growth impairment is mediated by two specific genes, parA and shi, on ICEclc, but its mechanistic details and cellular dynamics are still unknown. We here developed fluorescence reporter strains to monitor intracellular behavior of ParA and Shi proteins as well as host cellular proliferation at the single-cell level. Superresolution imaging revealed that ParA colocalized with the host nucleoid while Shi diffused in cytoplasm during the growth impairment. Mutations in the Walker A motif of ParA diminished the inhibitory effect. Combining quantitative time-lapse microscopy and numerical simulations using mathematical models, we found that ParA and Shi initially blocked cell division and then, as time elapsed, inhibited cellular elongation. The parA-shi locus is highly conserved in other ICEs, and the ParA-Shi-mediated growth inhibition was still observed in different proteobacterial species, suggesting that the ICEs have evolved the system to efficiently distribute themselves in the niche. 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.\n\nAuthor summaryHorizontal gene transfer is a major diving force for bacterial evolution, which is frequently mediated by mobile DNA vectors, such as plasmids and bacteriophages. Integrative and conjugative elements (ICEs) are a relatively new class of mobile vectors, which normally integrate 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 more abundant than plasmids among bacteria. Why so prevalent? One of the characteristics of ICEclc, an ICE model in proteobacteria, is that it develops specialized cells which entirely dedicate to the ICE horizontal transmission by repressing their proliferation. Here, we qualitatively and quantitatively describe two proteins, which are expressed from ICEclc when it transfers, and how they actually inhibit the host cell growth. Our results suggest that the system has evolved for the efficient horizontal transmission and is widely conserved in the ICE family.