Fig. 7.

Model of the regulation and roles of the P. aeruginosa choline transporters in an environment with an osmolarity that is physiologically relevant to a eukaryotic host. (A) Choline promotes expression of betT1, betT3, and cbcXVW. Increased expression in response to choline and GB is dependent on the regulators BetI and GbdR, respectively, and choline is converted into GB by BetAB (1). (B) Following cellular exposure to an abundance of choline (e.g., >1 mM), the initial transport depends primarily on BetT1 and BetT3, with BetT1 initially contributing more than BetT3, possibly because of higher BetT1 protein levels in uninduced cells. (C) Adaption to the presence of abundant choline involves increasing expression of betT1 and betT3, as shown in panel A, with increasing choline uptake promoting the induction of cbcXWV. The contribution of CbcXWV to the total choline uptake increases as its protein levels increase, while the contribution of BetT3 relative to BetT1 increases due to the greater transport capacity of BetT3 (Fig. 3). Growth data indicated that either BetT3 or CbcXWV is sufficient for robust growth with choline as the sole carbon source. (D) Exposure of cells to abundant GB prior to choline greatly increases the contribution of CbcXWV to choline transport due to cbcXWV induction, and this circumvents the major roles for BetT1 and BetT3 in transport immediately following exposure to choline. OM, outer membrane; IM, inner membrane.