Export of protein in Escherichia coli: a novel mutation in ompC affects expression of other major outer membrane proteins.

A mutation within the ompC structural gene of Escherichia coli K-12 which affects expression of outer membrane proteins was characterized. The mutation consisted of a 6-base-pair deletion near the 3' end of the gene which removed the amino acids Val-300 and Gly-301 of the mature coding sequence but otherwise left the reading frame intact. The deletion occurred within a region highly conserved among the porins. No protein product was detected from a single copy of the mutant gene. The mutation caused a trans-dominant decrease in the expression of a wild-type ompC allele. The mutation caused a similar decrease in the amounts of OmpA, OmpF, LamB, and Lc proteins, yet it did not appear to affect the minor outer membrane proteins. It had no significant effect on transcription from either ompF or ompC promoters as measured with lacZ operon fusions. The effects of the mutation on other proteins were completely eliminated when the signal sequence was disrupted so that the mutant protein no longer interacted with the secretion machinery of the cell but instead accumulated as precursor in the cytoplasm. A model is proposed involving the translocation of proteins to the outer membrane and the importance of protein conformation in this process.