DNA duplication in Burkholderia thailandensis induces biofilm formation by activating a two-component regulatory system.

Abstract

Burkholderia thailandensis strain E264 (BtE264) and close relatives stochastically duplicate a 208.6 kb region of chromosome I via RecA-dependent recombination between two nearly identical insertion sequence elements. Because homologous recombination occurs at a constant, low level, populations of BtE264 are always heterogeneous, but cells containing two or more copies of the region (Dup+) have an advantage, and hence predominate, during biofilm growth, while those with a single copy (Dup-) are favored during planktonic growth. Moreover, only Dup+ bacteria form 'efficient' biofilms within 24 hours in liquid medium. We determined that duplicate copies of a subregion containing genes encoding an archaic chaperone-usher pathway pilus (csuFABCDE) and a two-component regulatory system (bfmSR) are necessary and sufficient for generating efficient biofilms and for conferring a selective advantage during biofilm growth. BfmSR functionality is required, as deletion of either bfmS or bfmR, or a mutation predicted to abrogate phosphorylation of BfmR, abrogates biofilm formation. However, duplicate copies of the csuFABCDE genes are not required. Instead, we found that BfmSR controls expression of csuFABCDE and bfmSR by activating a promoter upstream of csuF during biofilm growth or when the 208.6 kb region, or just bfmSR, are duplicated. Single cell analyses showed that duplication of the 208.6 kb region is sufficient to activate BfmSR in 75% of bacteria during planktonic (BfmSR 'OFF') growth conditions. Together, our data indicate that the combination of deterministic two-component signal transduction and stochastic, duplication-mediated activation of that TCS form a bet-hedging strategy that allows BtE264 to survive when conditions shift rapidly from those favoring planktonic growth to those requiring biofilm formation, such as may be encountered in the soils of Southeast Asia and Northern Australia. Our data highlight the positive impact that transposable elements can have on the evolution of bacterial populations.