Characterization of the Escherichia coli protein-export gene secB

The Escherichia coli secB gene product is required for normal export of envelope proteins out of the cell cytoplasm. In this report, we present the identification and nucleotide sequence of the secB coding sequence. The secB structural gene overlaps almost completely with a predicted open reading frame (ORF) that is encoded on the opposite strand. To establish the identity of the secB ORF, we characterized a secB mutation that caused total loss of secB function, based upon its phenotype. This mutation resulted from a nucleotide change that caused an ochre mutation in one ORF (the secB gene) and a silent (no amino acid change) codon change in the opposite ORF.