YfiK from Escherichia coli Promotes Export of O -Acetylserine and Cysteine

Abstract

yfiK was discovered as a gene augmenting cysteine production when it was overexpressed in an industrial Escherichia coli production strain. The gene product is an integral membrane protein with about six predicted transmembrane helices; it belongs to the RhtB family of export proteins. YfiK overproduction from a plasmid leads to drastic and parallel secretion of O-acetylserine and cysteine into the medium but only when the organism possesses a serine transacetylase that is feedback insensitive to cysteine. Externally provided O-acetylserine obviated this requirement for cysteine secretion both in the yfiK-carrying transformant and in the wild type. A ⌬yfiK mutant did not show any phenotype, and it exported O-acetylserine and cysteine when transformed with a plasmid carrying ydeD, a previously characterized, alternate O-acetylserine/cysteine exporter. Since a ydeD-yfiK double mutant showed the same pattern, it appears that YfiK and YdeD act independently. The necessity for the cell to regulate the size of the internal pool of O-acetylserine via synthesis of exporter proteins could be connected to the fact that this compound (when supplied externally) inhibits growth. Overexpression of either ydeD or yfiK leads to alleviation of this inhibition paralled by increased resistance to azaserine, which is an analog of O-acetylserine.

An Excretory Function for theEscherichia coliOuter Membrane Pore TolC: Upregulation ofmarAandsoxSTranscription and Rob Activity Due to Metabolites Accumulated intolCMutants

“…Kawamura-Sato et al (21) have shown that AcrEF is important for indole excretion. Franke et al (14) have shown that YfiK plays a significant role in the excretion of cysteine-cystine, and Yamada et al (60) have shown that a number of other pumps, including some pumps known to be active in multidrug efflux, also are involved in cystine excretion. TolC, but not AcrAB, has been shown to also have a role in this process (58).…”

Characterization of the Escherichia coli AaeAB Efflux Pump: a Metabolic Relief Valve?

“…Thus, the Corynebacterium glutamicum LysE lysine effluxer (3,6), ThrE threonine and serine effluxer (40), and BrnFE isoleucine and leucine effluxer (17) and the E. coli RhtA, RhtB, and RhtC threonine effluxers (19,21,53) and ArgO arginine effluxer (26) may serve a role in pumping excess amino acids out to the cell. Similarly, the E. coli EamA (YdeD) (8) and EamB (YfiK) (11) transporters, which efflux cysteine pathway metabolites, may also play a role in ridding the cell of excess metabolites that FIG. 3.…”

Evidence for an Arginine Exporter Encoded by yggA ( argO ) That Is Regulated by the LysR-Type Transcriptional Regulator ArgP in Escherichia coli

“…However, an Arg efflux system is not known to exist in E. coli, and indeed only a few amino acid exporters have been characterized for any bacterium (reviewed in references 1, 9, and 17). The amino acid exporters so far identified for E. coli include RhtB and RhtC, for threonine and homoserine (27,43), and YdeD and YfiK, for cysteine and O-acetylserine (16,18). The exporter LysE, for both Lys and Arg, has been well characterized in Corynebacterium glutamicum, and the primary sequence of LysE is similar to the product of an E. coli open reading frame called yggA (3,7,8,42).…”