Determinants for overproduction of the Pseudomonas oleovorans cytoplasmic membrane protein alkane hydroxylase in alk+ Escherichia coli W3110

Response of soil bacterial communities to high petroleum content in the absence of remediation procedures

Oil spills are events that frequently lead to petroleum pollution. This pollution may cause stress to microbial communities, which require long adaption periods. Soil petroleum pollution is currently considered one of the most serious environmental problems. In the present work, processes occurring in the bacterial communities of three soil samples with different physicochemical characteristics, artificially polluted with 12% of crude oil, were investigated in 120-day laboratory experiment. It was found that the total petroleum hydrocarbon content did not decrease during this time; however, the proportion of petroleum fractions was altered. Petroleum pollution led to a short-term decrease in the bacterial 16S rRNA gene copy number. On the basis of amplicon sequencing analysis, it was concluded that bacterial community successions were similar in the three soils investigated. Thus, the phyla Actinobacteria and Proteobacteria and candidate TM7 phylum (Saccaribacteria) were predominant with relative abundances ranging from 35 to 58%, 25 to 30%, and 15 to 35% in different samples, respectively. The predominant operational taxonomic units (OTUs) after pollution belonged to the genera Rhodococcus and Mycobacterium, families Nocardioidaceae and Sinobacteraceae, and candidate class ТМ7-3. Genes from the alkIII group encoding monoxygenases were the most abundant compared with other catabolic genes from the alkI, alkII, GN-PAH, and GP-PAH groups, and their copy number significantly increased after pollution. The copy numbers of expressed genes involved in the horizontal transfer of catabolic genes, FlgC, TraG, and OmpF, also increased after pollution by 11-33, 16-63, and 11-71 times, respectively. The bacterial community structure after a high level of petroleum pollution changed because of proliferation of the cells that initially were able to decompose hydrocarbons, and in the second place, because proliferation of the cells that received these catabolic genes through horizontal transfer.

Flow-cytometric detection of changes in the physiological state of E. coli expressing a heterologous membrane protein during carbon-limited fedbatch cultivation

The key to optimizing productivity during industrial fermentations is the ability to rapidly monitor and interpret the physiological state of single microbial cells in a population and to recognize and characterize different sub-populations. Here, a flow cytometry-based method for the reproducible detection of changes in membrane function and/or structure of recombinant E. coli JM101 (pSPZ3) expressing xylene monooxygenase (XMO), was developed. XMO expression led to compromised but not permeabilized cell membranes. This was deduced from the fact that recombinant cells only stained with ethidium bromide (EB) and not with propidium iodide (PI). During the glucose-limited fedbatch cultivation, an increase from 25% to 95% of EB-stained cells was observed, occurring between 2 and 5 h after induction. Control experiments confirmed that this increase was due to the recombinant protein production and not caused by any possible effects of varying substrate availability, high cell density, plasmid replication or the presence of the inducing agent. We hypothesize that the integration of the recombinant protein into the cell membrane physically disrupted the functionality of the efflux pumps, thus resulting in EB-staining of the recombinant cells. This method enabled us to detect changes in the physiological state of single cells 2-4 h before other indications of partial cell damage, such as unbalanced growth, acetate accumulation and an increased CO(2) production rate, were observed. This method therefore shows promise with respect to the further development of an early-warning system to prevent sudden productivity decreases in processes with recombinant E. coli expressing heterologous membrane proteins.

The PalkBFGHJKL promoter is under carbon catabolite repression control in Pseudomonas oleovorans but not in Escherichia coli alk+ recombinants

The alk genes are located on the OCT plasmid of Pseudomonas oleovorans and encode an inducible pathway for the utilization of n-alkanes as carbon and energy sources. We have investigated the influence of alternative carbon sources on the induction of this pathway in P. oleovorans and Escherichia coli alk+ recombinants. In doing so, we confirmed earlier reports that induction of alkane hydroxylase activity in pseudomonads is subject to carbon catabolite repression. Specifically, synthesis of the monooxygenase component AlkB is repressed at the transcriptional level. The alk genes have been cloned into plasmid pGEc47, which has a copy number of about 5 to 10 per cell in both E. coli and pseudomonads. Pseudomonas putida GPo12 is a P. oleovorans derivative cured of the OCT plasmid. Upon introduction of pGEc47 in this strain, carbon catabolite repression of alkane hydroxylase activity was reduced significantly. In cultures of recombinant E. coli HB101 and W3110 carrying pGEc47, induction of AlkB and transcription of the alkB gene were no longer subject to carbon catabolite repression. This suggests that carbon catabolite repression of alkane degradation is regulated differently in Pseudomonas and in E. coli strains. These results also indicate that PalkBFGHJKL, the Palk promoter, might be useful in attaining high expression levels of heterologous genes in E. coli grown on inexpensive carbon sources which normally trigger carbon catabolite repression of native expression systems in this host.

Carbon-source-dependent expression of the PalkB promoter from the Pseudomonas oleovorans alkane degradation pathway

Pseudomonas oleovorans GPo1 can metabolize medium-chain-length alkanes by means of an enzymatic system whose induction is regulated by the AlkS protein. In the presence of alkanes, AlkS activates the expression of promoter PalkB, from which most of the genes of the pathway are transcribed. In addition, expression of the first enzyme of the pathway, alkane hydroxylase, is known to be influenced by the carbon source present in the growth medium, indicating the existence of an additional overimposed level of regulation associating expression of the alk genes with the metabolic status of the cell. Reporter strains bearing PalkB-lacZ transcriptional fusions were constructed to analyze the influence of the carbon source on induction of the PalkB promoter by a nonmetabolizable inducer. Expression was most efficient when cells grew at the expense of citrate, decreasing significantly when the carbon source was lactate or succinate. When cells were grown in Luria-Bertani rich medium, PalkB was strongly down-regulated. This effect was partially relieved when multiple copies of the gene coding for the AlkS activator were present and was not observed when the promoter was moved to Escherichia coli, a heterologous genetic background. Possible mechanisms responsible for PalkB regulation are discussed.

The AlkB monooxygenase of Pseudomonas oleovorans--synthesis, stability and level in recombinant Escherichia coli and the native host

We have studied the synthesis and stability of the monooxygenase AlkB of Pseudomonas oleovorans in its natural host and in recombinant Escherichia coli. Three strains were investigated: the prototype strain P. oleovorans and the E. coli alk+ recombinants HB101 (pGEc47) and W3110 (pGEc47). Plasmid pGEc47 allows regulated expression of alkB and synthesis of active AlkB in E. coli. The E. coli strains were selected because E. coli HB101 (pGEc47) produces similar amounts of AlkB as P. oleovorans (1.5-2% of total cell protein), whereas E. coli W3110 (pGEc47) is able to make substantially (about fivefold) more AlkB. The AlkB synthesis and degradation rates in batch cultures of the three strains were determined by means of isotopic-labeling and immunological techniques. The mean specific AlkB synthesis rates in P. oleovorans, E. coli HB101 (pGEc47) and E. coli W3110 (pGEc47) were approximately 7, 12.5 and 45 microg x mg protein(-1) x h(-1), respectively. The half-lives of AlkB were estimated to be 80, 3 and 15 for P. oleovorans, E. coli HB101 (pGEc47) and E. coli W3110 (pGEc47), respectively. Thus, the intracellular AlkB level in each of the three strains was the result of their AlkB synthesis and degradation rates. The AlkB level during batch growth was modelled by means of experimentally derived parameters for AlkB synthesis and degradation, and showed good agreement with AlkB levels determined by means of immunoblotting in all strains investigated.