Bacillus competence pili are efficient single - and double stranded DNA uptake machines

Natural competence describes the ability of diverse bacterial species to take up DNA from the environment for integration into the host chromosome, allowing the acquisition of novel genetic information. Uptake in many species operates via a dynamic pilus structure that pulls DNA through the cell wall into the periplasm by pilus retraction, followed by transport of one of the two DNA strands into the cytosol by an active membrane transporter. How double stranded DNA with a bending contour of hundreds of base pairs can fit through a narrow pore accommodating a pilus has been a puzzling question. We show that purified major and minor pilins from two Bacillus species, B. subtilis and Geobacillus thermodenitrificans, show efficient binding to double as well as to single stranded DNA with similar affinities. Consistent with uptake, ssDNA accumulates and moves within the periplasm before transport through the cell membrane, with similar dynamics as dsDNA. Thus, Bacillus competence pili are dsDNA as well as ssDNA uptake machines. The binding of pili to ssDNA regions within environmental DNA likely increases the efficiency of competent bacteria to pull extended DNA strands through the cell wall. Uptake of environmental ssDNA may also provide a beneficial source of nutrition for bacteria during stationary growth phase.