2.A.124 The Lysine Exporter (LysO) Family
In E. coli, argO encodes an exporter for l-arginine (Arg) and its toxic analogue canavanine (CAN). It's transcriptional activation and repression occurs in response to Arg and l-lysine (Lys), respectively, mediated by the regulator ArgP. Accordingly, argO and argP mutants are CAN supersensitive (CAN(ss)). ybjE encodes a Lys exporter as reported by Ueda and coworkers (T. Ueda, Y. Nakai, Y. Gunji, R. Takikawa, and Y. Joe, U.S. patents 7,629,142 B2 [December 2009] and 8,383,363 B1 [February 2013] and European patent 1,664,318 B1 [September 2009]). ybjE was isolated as a multicopy suppressor of the CAN(ss) phenotype of a strain lacking ArgO (Pathania and Sardesai 2015). The absence of YbjE did not confer a CAN(ss) phenotype but instead conferred hypersensitivity to the lysine antimetabolite, thialysine, and led to growth inhibition by the dipeptide lysylalanine, due to elevated cellular Lys content. YbjE overproduction resulted in Lys excretion and syntrophic cross-feeding of a Lys auxotroph. Constitutive overexpression of argO promoted Lys cross-feeding that is indicative of a latent Lys export potential of ArgO. Arg modestly repressed ybjE transcription in an ArgR-dependent manner, and ArgR displayed Arg-sensitive binding to the ybjE promoter region in vitro. The reciprocal repression of argO and ybjE, respectively, by Lys and Arg confers the specificity for basic amino acid export by distinct paths. Such cross-repression contributes to maintenance of cytoplasmic Arg/Lys balance (Pathania and Sardesai 2015).
The generalized reaction catalyzed by LysO) is:
Lysine (in) + H+ (out) → Lysine (out) + H+ (in).