OVERPRODUCTION OF NLPE, A NEW OUTER-MEMBRANE LIPOPROTEIN, SUPPRESSES THE TOXICITY OF PERIPLASMIC LACZ BY ACTIVATION OF THE CPX SIGNAL-TRANSDUCTION PATHWAY

The LamB-LacZ-PhoA tripartite fusion protein is secreted to the peripl asm, where it exerts a toxicity of unknown origin during high-level sy nthesis in the presence of the inducer maltose, a phenotype referred t o as maltose sensitivity. We selected multicopy suppressors of this to xicity that allow growth of the tripartite fusion strains in the prese nce of maltose. Mapping and subclone analysis of the suppressor locus identified a previously uncharacterized chromosomal region at 4.7 min that is responsible for suppression. DNA sequence analysis revealed a new gene with the potential to code for a protein of 236 amino acids w ith a predicted molecular mass of 25,829 Da. The gene product contains an amino-terminal signal sequence to direct the protein for secretion and a consensus lipoprotein modification sequence. As predicted from the sequence, the suppressor protein is labeled with [H-3]palmitate an d is localized to the outer membrane. Accordingly, the gene has been n amed nlpE (for new lipoprotein E). Increased expression of NlpE suppre sses the maltose sensitivity of tripartite fusion strains and also the extracytoplasmic toxicities conferred by a mutant outer membrane prot ein, LamBA23D. Suppression occurs by activation of the Cpx two-compone nt signal transduction pathway. This pathway controls the expression o f the periplasmic protease DegP and other factors that can combat cert ain types of extracytoplasmic stress.