Identification of a major facilitator protein from <i>Escherichia coli</i> involved in efflux of metabolites of the cysteine pathway

<jats:p>A chromosomal fragment has been identified in a gene bank from <jats:italic>Escherichia coli</jats:italic>, which augmented the yield of cysteine in an industrial production strain. Subcloning and genetic analysis showed that an open reading frame coding for a product of 299 amino acids (Orf299) was responsible. Orf299 was synthesized in the T7 polymerase/promoter system and exhibited the properties of an integral membrane protein. Mutational interruption of <jats:italic>orf299</jats:italic> did not cause a distinct phenotype; however, transformants overexpressing <jats:italic>orf299</jats:italic> had lost the ability to grow in minimal medium unless it was supplemented with a source of reduced sulphur compounds, and they excreted considerable amounts of cysteine and O‐acetyl‐<jats:sc>l</jats:sc>‐serine, especially in the presence of thiosulphate. Most of the cysteine was found to be masked in 2‐methyl‐2,4‐thiazolidinedicarboxylic acid. N‐acetyl‐<jats:sc>l</jats:sc>‐serine was also present in the medium, but it is open to question whether it represents a primary excretion product. Measurement of the induction status of the cysteine regulon by means of a <jats:italic>cysK</jats:italic>′–′<jats:italic>lacZ</jats:italic> gene fusion demonstrated that the regulon is not induced upon growth in the presence of a poor sulphur source and that the introduction of a constitutive <jats:italic>cysB</jats:italic> allele alleviates this deficiency. The results indicate that <jats:italic>orf299</jats:italic> codes for an export pump for different metabolites of the cysteine pathway. Its relation to other efflux systems and the physiological role are discussed.</jats:p>