Genetic control of ColE1 plasmid stability that is independent of plasmid copy number regulation.

ColE1-like plasmid vectors are widely used for expression of recombinant genes in E. coli. For these vectors, segregation of individual plasmids into daughter cells during cell division appears to be random, making them susceptible to loss over time when no mechanisms ensuring their maintenance are present. Here we use the plasmid pGFPuv in a recA relA strain as a sensitized model to study factors affecting plasmid stability in the context of recombinant gene expression. We find that in this model, plasmid stability can be restored by two types of genetic modifications to the plasmid origin of replication (ori) sequence: point mutations and a novel 269 nt duplication at the 5' end of the plasmid ori, which we named DAS (duplicated anti-sense) ori. Combinations of these modifications produce a range of copy numbers and of levels of recombinant expression. In direct contradiction with the classic random distribution model, we find no correlation between increased plasmid copy number and increased plasmid stability. Increased stability cannot be explained by reduced levels of recombinant gene expression either. Our observations would be more compatible with a hybrid clustered and free-distribution model, which has been recently proposed based on detection of individual plasmids in vivo using super-resolution fluorescence microscopy. This work suggests a role for the plasmid ori in the control of segregation of ColE1 plasmids that is distinct from replication initiation, opening the door for the genetic regulation of plasmid stability as a strategy aimed at enhancing large-scale recombinant gene expression or bioremediation.

Mycobacterium smegmatis RoxY is a repressor of oxyS and contributes to resistance to oxidative stress and bactericidal ubiquitin-derived peptides.

The mycobactericidal properties of macrophages include the generation of reactive oxygen intermediates and the delivery of bacteria to a hydrolytic lysosome enriched in bactericidal ubiquitin-derived peptides (Ub-peptides). To better understand the interactions of ubiquitin-derived peptides with mycobacteria and identify putative mycobacterial intrinsic resistance mechanisms, we screened for transposon mutants with increased susceptibility to the bactericidal Ub-peptide Ub2. We isolated 27 Mycobacterium smegmatis mutants that were hypersusceptible to Ub2. Two mutants were isolated that possessed mutations in the msmeg_0166 gene, which encodes a transcriptional regulator. The msmeg_0166 mutants were also hypersusceptible to other host antimicrobial peptides and oxidative stress. In characterizing msmeg_0166, we found that it encodes a repressor of oxyS, and therefore we have renamed the gene roxY. We demonstrate that RoxY and OxyS contribute to M. smegmatis resistance to oxidative stress. An ahpD transposon mutant was also isolated in our screen for Ub-peptide hypersusceptibility. Overexpression of oxyS in M. smegmatis reduced transcription of the ahpCD genes, which encode a peroxide detoxification system. Our data indicate that RoxY, OxyS, and AhpD play a role in the mycobacterial oxidative stress response and are important for resistance to host antimicrobial peptides.

Expression of Pseudomonas stutzeri Zobell cytochrome c-551 and its H47A variant in Escherichia coli.

The nirM gene encoding cytochrome c-551 from Pseudomonas stutzeri Zobell (PZ) has been expressed in Escherichia coli at levels higher than those previously reported but only under strict anaerobic growth conditions. Expression yields for wild-type cytochrome in this study typically reached 0.6 micromol per liter of saturated E. coli culture (5.5mg/L). Culture conditions investigated are compared to obtained c-551 expression levels; the results may lead to a greater understanding of the challenges encountered when expressing c-type hemoproteins in E. coli. The nirM gene was mutated to produce a histidine-47-alanine mutation of c-551 that been heterologously expressed in E. coli using optimum culture conditions and had its physiochemical properties compared to those of the wild-type protein. In PZ, the histidine-47 residue is part of a conserved hydrogen-bonding network located at the bottom of the heme crevice that also involves tryptophan-56 and a heme propionate. Ionization events within this network are experimentally demonstrated to modulate c-551 oxidation-reduction potential and its observed dependence on pH around neutrality. The redox potential of the mutant cytochrome still displays pH-dependence; however, the midpoint potential is approximately 25mV lower with respect to wild-type c-551 at neutral pH while the pK at which the heme propionate (HP-17) ionizes is lowered by 1.3 pH units. Temperature and chemical denaturant studies also show that loss of the hydrogen-bond-donating imidazole leads to a large decrease in c-551 tertiary stability.