Two distinct regions of the LamB signal sequence function in different steps in export

Predictability of Phenotype in Relation to Common β-Lactam Resistance Mechanisms in Escherichia coli and Klebsiella pneumoniae

The minimal concentration of antibiotic required to inhibit the growth of different isolates of a given species with no acquired resistance mechanisms has a normal distribution. We have previously shown that the presence or absence of transmissible antibiotic resistance genes has excellent predictive power for phenotype. In this study, we analyzed the distribution of six β-lactam antibiotic susceptibility phenotypes associated with commonly acquired resistance genes in Enterobacteriaceae in Sydney, Australia. Escherichia coli (n = 200) and Klebsiella pneumoniae (n = 178) clinical isolates, with relevant transmissible resistance genes (blaTEM, n = 33; plasmid AmpC, n = 69; extended-spectrum β-lactamase [ESBL], n = 116; and carbapenemase, n = 100), were characterized. A group of 60 isolates with no phenotypic resistance to any antibiotics tested and carrying none of the important β-lactamase genes served as comparators. The MICs for all drug-bacterium combinations had a normal distribution, varying only in the presence of additional genes relevant to the phenotype or, for ertapenem resistance in K. pneumoniae, with a loss or change in the outer membrane porin protein OmpK36. We demonstrated mutations in ompK36 or absence of OmpK36 in all isolates in which reduced susceptibility to ertapenem (MIC, >1 mg/liter) was evident. Ertapenem nonsusceptibility in K. pneumoniae was most common in the context of an OmpK36 variant with an ESBL or AmpC gene. Surveillance strategies to define appropriate antimicrobial therapies should include genotype-phenotype relationships for all major transmissible resistance genes and the characterization of mutations in relevant porins in organisms, like K. pneumoniae.

Lon protease quality control of presecretory proteins in Escherichia coli and its dependence on the SecB and DnaJ (Hsp40) chaperones

Various environmental insults result in irreversible damage to proteins and protein complexes. To cope, cells have evolved dedicated protein quality control mechanisms involving molecular chaperones and proteases. Here, we provide both genetic and biochemical evidence that the Lon protease and the SecB and DnaJ/Hsp40 chaperones are involved in the quality control of presecretory proteins in Escherichia coli. We showed that mutations in the lon gene alleviate the cold-sensitive phenotype of a secB mutant. Such suppression was not observed with either clpP or clpQ protease mutants. In comparison to the respective single mutants, the double secB lon mutant strongly accumulates aggregates of SecB substrates at physiological temperatures, suggesting that the chaperone and the protease share substrates. These observations were extended in vitro by showing that the main substrates identified in secB lon aggregates, namely proOmpF and proOmpC, are highly sensitive to specific degradation by Lon. In contrast, both substrates are significantly protected from Lon degradation by SecB. Interestingly, the chaperone DnaJ by itself protects substrates better from Lon degradation than SecB or the complete DnaK/DnaJ/GrpE chaperone machinery. In agreement with this finding, a DnaJ mutant protein that does not functionally interact in vivo with DnaK efficiently suppresses the SecB cold-sensitive phenotype, highlighting the role of DnaJ in assisting presecretory proteins. Taken together, our data suggest that when the Sec secretion pathway is compromised, a pool of presecretory proteins is transiently maintained in a translocation-competent state and, thus, protected from Lon degradation by either the SecB or DnaJ chaperones.

PrlA and PrlG suppressors reduce the requirement for signal sequence recognition

Selection for suppressors of defects in the signal sequence of secretory proteins has led most commonly to identification of prlA alleles and less often to identification of prlG alleles. These genes, secY/prlA and secE/prlG, encode integral membrane components of the protein translocation system of Escherichia coli. We demonstrate that an outer membrane protein, LamB, that lacks a signal sequence can be exported with reasonable efficiency in both prlA and prlG suppressor strains. Although the signal sequence is not absolutely required for export of LamB, the level of export in the absence of prl suppressor alleles is exceedingly low. Such strains are phenotypically LamB-, and functional LamB can be detected only by using sensitive infectious-center assays. Suppression of the LamB signal sequence deletion is dependent on normal components of the export pathway, indicating that suppression is not occurring through a bypass mechanism. Our results indicate that the majority of the known prlA suppressors function by an identical mechanism and, further, that the prlG suppressors work in a similar fashion. We propose that both PrlA and PrlG suppressors lack a proofreading activity that normally rejects defective precursors from the export pathway.

A new suppressor of a lamB signal sequence mutation, prlZ1, maps to 69 minutes on the Escherichia coli chromosome

Reversion analysis has been employed to isolate suppressors that restore export of a unique LamB signal sequence mutant. The mutation results in a substitution of Arg for Met at position 19, which prevents LamB export to the outer membrane and leads to a Dex- phenotype. Unlike other LamB signal sequence mutants utilized for reversion analysis, LamB19R becomes stably associated with the inner membrane in an export-specific manner. In this study, Dex+ revertants were selected and various suppressors were isolated. One of the extragenic suppressors, designated prlZ1, was chosen for further study. prlZ1 maps to 69 min on the Escherichia coli chromosome. The suppressor is dominant and SecB dependent. In addition to its effect on lamB19R, prlZ1 suppresses the export defect of signal sequence point mutations at positions 12, 15, and 16, as well as several point mutations in the maltose-binding protein signal sequence. prlZ1 does not suppress deletion mutations in either signal sequence. This pattern of suppression can be explained by interaction of a helical LamB signal sequence with the suppressor.