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 DeltayfiK 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.