Single molecule dynamics of DNA receptor ComEA, membrane permease ComEC and taken up DNA in competent Bacillus subtilis cells

Abstract

In competent gram-negative and gram-positive bacteria, double stranded DNA is taken up through the outer cell membrane and/or the cell wall, and is bound by ComEA, which in Bacillus subtilis is a membrane protein. DNA is converted to single stranded DNA, and transported through the cell membrane via ComEC. We show that in Bacillus subtilis, the C-terminus of ComEC, thought to act as a nuclease, is not only important for DNA uptake, as judged from a loss of transformability, but also for the localization of ComEC to the cell pole and its mobility within the cell membrane. Using single molecule tracking, we show that only 13% of ComEC molecules are statically localised at the pole, while 87% move throughout the cell membrane. These experiments suggest that recruitment of ComEC to the cell pole is mediated by a diffusion/capture mechanism. Mutation of a conserved aspartate residue in the C-terminus, likely affecting metal binding, strongly impairs transformation efficiency, suggesting that this periplasmic domain of ComEC could indeed serve a catalytic function as nuclease. By tracking fluorescently labeled DNA, we show that taken up DNA has a similar mobility within the periplasm as ComEA, suggesting that most taken up molecules are bound to ComEA. We show that DNA can be highly mobile within the periplasm, indicating that this subcellular space can act as reservoir for taken up DNA, before its entry into the cytosol. ImportanceBacteria can take up DNA from the environment and incorporate it into their chromosome in case similarity to the genome exists. This process of "natural competence" can result in the uptake of novel genetic information leading to horizontal gene transfer. We show that fluorescently labelled DNA moves within the periplasm of competent Bacillus subtilis cells with similar dynamics as DNA receptor ComEA, and thus takes a detour to get stored before uptake across the cell membrane into the cytosol by DNA permease ComEC. The latter assembles at a single cell pole, likely by a diffusion-capture mechanism, and requires its large C-terminus, including a conserved residue thought to confer nuclease function, for proper localization, function and mobility within the membrane.