A defined mutation in the protein export gene within the spc ribosomal protein operon of Escherichia coli: isolation and characterization of a new temperature-sensitive secY mutant.

We describe the properties of a temperature-sensitive mutant, ts24, of Escherichia coli. The mutant has a conditional defect in export of periplasmic and outer membrane proteins. At 42 degrees C, precursor forms of these proteins accumulate within the cell where they are protected from digestion by externally added trypsin. The accumulated precursors are secreted and processed very slowly at 42 degrees C. The mutation is complemented by expression of the wild-type secY (or prlA) gene, which has been cloned into a plasmid vector from the promoter-distal part of the spc ribosomal protein operon. The mutant has a single base change in the middle of the secY gene, which would result in the replacement of a glycine residue by aspartic acid in the protein product. These results demonstrate that the gene secY (prlA) is essential for protein translocation across the E. coli cytoplasmic membrane.