Activation of the integrative and conjugative element Tn916 causes growth arrest and death of host bacteria

Integrative and conjugative elements (ICEs) serve as major drivers of bacterial evolution. These elements often confer some benefit to a host cell, including antibiotic resistance, metabolic capabilities, or pathogenic determinants. ICEs can also have negative impacts on their host cells. Here, we investigated the effects of the ICE (conjugative transposon) Tn916 on host cells. Because Tn916 is active in a relatively small subpopulation of host cells, we developed a fluorescent reporter system for monitoring activation of Tn916 in single cells. We found that when active in Bacillus subtilis and its natural host Enterococcus faecalis, Tn916 inhibited cell division and most cells died. We also observed these phenotypes on the population level in B. subtilis utilizing a modified version of Tn916 that can be activated in the majority of cells. We identified two genes (orf17 and orf16) in Tn916 that were sufficient to cause host growth defects and identified a single gene, yqaR, that is found in a defective phage (skin) in the B. subtilis chromosome that is required for this phenotype. However, these three genes are only partially responsible for the growth defect caused by Tn916, indicating that Tn916 possesses multiple mechanisms to affect growth and viability of host cells. These results highlight the complex relationships that conjugative elements have with their host cells and the interplay between mobile genetic elements.