Optimization of the inefficient translation initiation region of the cpxP gene from Escherichia coli.

The Escherichia coli Cpx envelope stress system is comprised of three proteins; the periplasmic regulatory CpxP, the inner membrane sensor kinase CpxA, and the cytoplasmic transcriptional activator CpxR. Although misfolded envelope proteins activate the Cpx system, the molecular mechanism by which this signal is sensed remains largely unknown. In an attempt to reconstitute the Cpx system from purified proteins, we failed to produce the small CpxP protein in its natural periplasmic compartment, but a high protein level was achieved when it was produced in the cytoplasm. Silent base mutations in the first codons of the cpxP gene encoding the signal sequence or substitution by two well-characterized signal sequences, those of MalE and DsbA, resulted in a large increase of the CpxP level in the periplasm. Our results support the hypothesis that periplasmic expression could be inhibited by sequence elements in the early coding signal sequence region of cpxP.