Mutation that suppresses the protein export defect of the secY mutation and causes cold-sensitive growth of Escherichia coli.

A cold-sensitive mutant was isolated among temperature-resistant revertants of the secY24 mutant defective in secretion of envelope proteins across the cytoplasmic membrane at 42 degrees C. A single mutation, designated ssyA3, is responsible both for the extragenic suppression of secY and for the cold-sensitive growth. In contrast to the parental secY24 mutant, the suppressed cells do not accumulate precursors of envelope proteins at any temperatures. The cells containing the ssyA3 mutation, whether in combination with secY24 or not, show an optimal growth at 42 degrees C and a very poor growth at 30 degrees C. At the low temperature, protein synthesis is generally slowed down, probably at the step of chain elongation. The gene ssyA was mapped at a new locus between hisS and glyA on the chromosome. It is possible that the product of this gene interacts both with the protein secretion system and the protein synthesizing system.