The secD locus of E.coli codes for two membrane proteins required for protein export

Cold-sensitive mutations in the secD locus of Escherichia coli result in severe defects in protein export at the non-permissive temperature of 23 degrees C. DNA sequence of a cloned fragment that includes the secD locus reveals open reading frames for seven polypeptide chains. Both deletions and TnphoA insertions in this clone have been used in maxicell and complementation studies to define the secD locus and its products. The secD mutations fall into two complementation groups, defining genes we have named secD and secF. These two genes comprise an operon, the first case of two genes involved in the export process being co-transcribed. The DNA sequence of the two genes along with alkaline phosphatase fusion analysis indicates that they code for integral proteins of the cytoplasmic membrane. We suggest that these two proteins may form a complex in the membrane which acts at late steps in the export process.