TABLE 1.

Strains and plasmids used in this study^a

Strain	Relevant genotype	Reference
ECL525	MC4100 Δ(argF-lac)U169 araD139 Δfrd-101 rpsL150 relA1 deoC1 flb-5301 ptsF25	22
JP406	ECL525 F′pOXgen	This study
ECL1212	JP406 ΔcpxRA-2	This study
ECL1215	JP406 ara+	34
JP408	JP406 zii-510::Tn10 cpxA9*	This study
JP466	JP406 argE::Tn10	34
JP467	JP406 argE::Tn10 cpxA2.1*	34

^aAll of the strains used in this study are isogenic derivatives of ECL525. F′pOXgen (14) was used to introduce conjugative ability to ECL525. The defined deletion ΔcpxRA-2 was constructed as described by Blum et al. (6). The 1.2-kb deletion between the XhoI and EcoRI sites in the cpxRA operon removed most of the coding sequence of the two genes. The deletion was confirmed by PCR. Mutant cpxA alleles were introduced into strain JP406 by phage P1 cotransduction of linked zii-510::Tn10 or argE::Tn10. Strain JP408 received the cpxA9* allele from strain AE2293, which was provided by P. M. Silverman (38). The cpxA9* mutant was originally selected for amikacin resistance (38) and shown to have a Leu38→Phe (TTT) substitution in the periplasmic domain of CpxA (42). This mutation was confirmed in strain JP408 by DNA sequencing. The cpxA2.1* mutant was isolated in a selection for increased expression of a lacZ transcriptional fusion to the yajC-secDF operon. This cpxA allele was sequenced and shown to have a Val20→Ala (GCG) substitution plus an insertion of Leu and Val (CTGGTG) between Ala20 and Leu21 in the first membrane-spanning segment. For other characteristics of this mutation, see reference 34. Strains with cpxA* alleles were routinely grown at 30°C, except where indicated, to minimize reversion or suppression.