A simple procedure for cloning and stable insertion of foreign genes into the chromosomes of gram-negative eubacteria was developed by combining in two sets of plasmids (i) the transposition features of Tn10 and Tn5; (ii) the resistances to the herbicide bialaphos, to mercuric salts and organomercurial compounds, and to arsenite, and (iii) the suicide delivery properties of the R6K-based plasmid pGP704. The resulting constructions contained unique NotI or SfiI sites internal to either the Tn10 or the Tn5 inverted repeats. These sites were readily used for cloning DNA fragments with the help of two additional specialized cloning plasmids, pUC18Not and pUC18Sfi. The newly derived constructions could be maintained only in donor host strains that produce the R6K-specified pi protein, which is an essential replication protein for R6K and plasmids derived therefrom. Donor plasmids containing hybrid transposons were transformed into a specialized lambda pir lysogenic Escherichia coli strain with a chromosomally integrated RP4 that provided broad-host-range conjugal transfer functions. Delivery of the donor plasmids into selected host bacteria was accomplished through mating with the target strain. Transposition of the hybrid transposon from the delivered suicide plasmid to a replicon in the target cell was mediated by the cognate transposase encoded on the plasmid at a site external to the transposon. Since the transposase function was not maintained in target cells, such cells were not immune to further transposition rounds. Multiple insertions in the same strain are therefore only limited by the availability of distinct selection markers. The utility of the system was demonstrated with a kanamycin resistance gene as a model foreign insert into Pseudomonas putida and a melanin gene from Streptomyces antibioticus into Klebsiella pneumoniae. Because of the modular nature of the functional parts of the cloning vectors, they can be easily modified and further selection markers can be incorporated. The cloning system described here will be particularly useful for the construction of hybrid bacteria that stably maintain inserted genes, perhaps in competitive situations (e.g., in open systems and natural environments), and that do not carry antibiotic resistance markers characteristic of most available cloning vectors (as is currently required of live bacterial vaccines).

The ‘Standard European Vector Architecture’ database (SEVA-DB, http://seva.cnb.csic.es) was conceived as a user-friendly, web-based resource and a material clone repository to assist in the choice of optimal plasmid vectors for de-constructing and re-constructing complex prokaryotic phenotypes. The SEVA-DB adopts simple design concepts that facilitate the swapping of functional modules and the extension of genome engineering options to microorganisms beyond typical laboratory strains. Under the SEVA standard, every DNA portion of the plasmid vectors is minimized, edited for flaws in their sequence and/or functionality, and endowed with physical connectivity through three inter-segment insulators that are flanked by fixed, rare restriction sites. Such a scaffold enables the exchangeability of multiple origins of replication and diverse antibiotic selection markers to shape a frame for their further combination with a large variety of cargo modules that can be used for varied end-applications. The core collection of constructs that are available at the SEVA-DB has been produced as a starting point for the further expansion of the formatted vector platform. We argue that adoption of the SEVA format can become a shortcut to fill the phenomenal gap between the existing power of DNA synthesis and the actual engineering of predictable and efficacious bacteria.

The itinerary followed by Pseudomonas putida from being a soil-dweller and plant colonizer bacterium to become a flexible and engineer-able platform for metabolic engineering stems from its natural lifestyle, which is adapted to harsh environmental conditions and all sorts of physicochemical stresses. Over the years, these properties have been capitalized biotechnologically owing to the expanding wealth of genetic tools designed for deep-editing the P. putida genome. A suite of dedicated vectors inspired in the core tenets of synthetic biology have enabled to suppress many of the naturally-occurring undesirable traits native to this species while enhancing its many appealing properties, and also to import catalytic activities and attributes from other biological systems. Much of the biotechnological interest on P. putida stems from the distinct architecture of its central carbon metabolism. The native biochemistry is naturally geared to generate reductive currency [i.e., NAD(P)H] that makes this bacterium a phenomenal host for redox-intensive reactions. In some cases, genetic editing of the indigenous biochemical network of P. putida (cis-metabolism) has sufficed to obtain target compounds of industrial interest. Yet, the main value and promise of this species (in particular, strain KT2440) resides not only in its capacity to host heterologous pathways from other microorganisms, but also altogether artificial routes (trans-metabolism) for making complex, new-to-Nature molecules. A number of examples are presented for substantiating the worth of P. putida as one of the favorite workhorses for sustainable manufacturing of fine and bulk chemicals in the current times of the 4th Industrial Revolution. The potential of P. putida to extend its rich native biochemistry beyond existing boundaries is discussed and research bottlenecks to this end are also identified. These aspects include not just the innovative genetic design of new strains but also the incorporation of novel chemical elements into the extant biochemistry, as well as genomic stability and scaling-up issues.

Summary: The genome of the soil bacterium Pseudomonas putida strain KT2440 has been erased of various determinants of resistance to antibiotics encoded in its extant chromosome. To this end, we employed a coherent genetic platform that allowed the precise deletion of multiple genomic segments in a large variety of Gram‐negative bacteria including (but not limited to) P. putida . The method is based on the obligatory recombination between free‐ended homologous DNA sequences that are released as linear fragments generated upon the cleavage of the chromosome with unique I‐SceI sites, added to the segment of interest by the vector system. Despite the potential for a SOS response brought about by the appearance of double stranded DNA breaks during the process, fluctuation experiments revealed that the procedure did not increase mutation rates – perhaps due to the protection exerted by I‐SceI bound to the otherwise naked DNA termini. With this tool in hand we made sequential deletions of genes mexC, mexE, ttgA and ampC in the genome of the target bacterium, orthologues of which are known to determine various degrees of antibiotic resistance in diverse microorganisms. Inspection of the corresponding phenotypes demonstrated that the efflux pump encoded by ttgA sufficed to endow P. putida with a high‐level of tolerance to β‐lactams, chloramphenicol and quinolones, but had little effect on, e.g. aminoglycosides. Analysis of the mutants revealed also a considerable diversity in the manifestation of the resistance phenotype within the population and suggested a degree of synergism between different pumps. The directed edition of the P. putida chromosome shown here not only enhances the amenability of this bacterium to deep genomic engineering, but also validates the corresponding approach for similar handlings of a large variety of Gram‐negative microorganisms.