Control of plasmid R6K copy numbers in isogenic rep+ and rep Escherichia coli strains

Analysis of growth rate effects on productivity of recombinant Escherichia coli populations using molecular mechanism models

The influence of growth rate on Escherichia coli plasmid content and expression of a cloned-gene product has been described by a mathematical model based upon the molecular mechanism of lambdadv plasmid replication and known relationships between growth rate and transcription and translation activities of the host cell. The model simulates correctly decreases in plasmid content with increasing growth rate as observed experimentally for pBR322, NR1, R1, and Col E1 plasmids. A maximum with respect to growth rate in intracellular product accumulation is indicated by the model, as is a transient overshoot in product concentration following a shift from smaller to larger growth rate. Available data, although very limited, show the same trends. These results, obtained without parameter or kinetic form adjustments or manipulation, clearly illustrate the advantages of kinetic descriptions of recombinant systems based upon the pertinent molecular mechanisms.

Analysis of growth rate effects on productivity of recombinant Escherichia coli populations using molecular mechanism models. Reprinted from Biotechnology and Bioengineering, Vol. 26, Issue 1, Pages 66-73 (1984)

The influence of growth rate on Escherichia coli plasmid content and expression of a cloned-gene product has been described by a mathematical model based upon the molecular mechanism of lambdadv plasmid replication and known relationships between growth rate and transcription and translation activities of the host cell. The model simulates correctly decreases in plasmid content with increasing growth rate as observed experimentally for pBR322, NR1, R1, and Col E1 plasmids. A maximum with respect to growth rate in intracellular product accumulation is indicated by the model, as is a transient overshoot in product concentration following a shift from smaller to larger growth rate. Available data, although very limited, show the same trends. These results, obtained without parameter or kinetic form adjustments or manipulation, clearly illustrate the advantages of kinetic descriptions of recombinant systems based upon the pertinent molecular mechanisms.

Further characterization of repair of 8-methoxypsoralen crosslinks in UV-excision-defective Escherichia coli

Evidence was previously presented for a new pathway for the repair of 8-methoxypsoralen DNA crosslinks. The pathway, which is independent of the uvrA gene but deficient in rep mutants, has now been further characterized and shown to be more active in minimal than in nutrient growth media and to be inhibited by acriflavine. Although crosslink repair is much reduced in recA bacteria, some still occurs as judged by the effect of acriflavine. By the same criterion, crosslink repair occurs in bacteria with point mutations in the uvrA and uvrB genes, in bacteria with a deletion covering the uvrB gene, and in polA uvrA bacteria. Bacteria with insertions rather than point mutations in the uvrA gene, although showing evidence of repair, demonstrated minimal inhibition with acriflavine suggesting the possibility that the uvrA gene product, even if enzymically inactive, might be able to interact with DNA lesions in the presence of acriflavine and prevent crosslink repair. Crosslink repair in E. coli WP2 uvrA is associated with base-pair substitution mutagenesis and may be characterized as an error-prone process. Crosslink repair in uvrA bacteria is reduced but not eliminated by a mutation in the umuC gene.

A new pathway for repair of cross-linkable 8-methoxypsoralen mono-adducts in Uvr strains of Escherichia coli

UvrA-deficient Escherichia coli bacteria possess a DNA repair pathway for certain photoinduced mono-adducts of 8-methoxypsoralen. Such adducts can be converted to highly lethal cross-links by a second exposure to near-UV light in the absence of 8-methoxypsoralen. If a period of incubation is allowed to intervene between the primary and cross-linking exposure, the lethal effect is considerably diminished, indicating that some of the mono-adducts become unavailable for cross-linking. A repair pathway is postulated which is pre-replicative, unaffected by chloramphenicol, acriflavine or caffeine, and inhibited at 0 degrees C. It is unaffected by a rep mutation but is blocked by a polA mutation, suggesting that it involves DNA strand breakage and at least some exonuclease action.

Influence of mutations at the rep gene on survival of Escherichia coli following ultraviolet light irradiation or 8-methoxypsoralen photosensitization: evidence for a recA+ rep+-dependent pathway for repair of DNA crosslinks

Bacteria mutant at the rep gene (specifying a DNA-unwinding enzyme) were slightly more sensitive than rep+ bacteria to far ultraviolet light (ca. 254 nm; (FUV) and to monofunctional psoralen photoproducts produced with near ultraviolet light (ca. 360 nm; NUV). The enhanced sensitivity was shown in uvrA excision-deficient bacteria but not in those carrying the recA mutation. It is concluded that the rep unwinding enzyme has a small promoting effect on post-replication recombination repair. Rep- bacteria were rather more sensitive to the DNA cross-linking action f 8-methoxypsoralen (8-MOP) and NUV. This effect also was found in uvrA but not in recA bacteria. A cross-link repair pathway, dependent on both the recA+ and rep+ genes, therefore exists in addition to the pathway which depends upon the uvrAB endonuclease. Various lines of evidence suggest that the pathway operates at or after DNA replication. Exposure of rep+ bacteria to 8-MOP plus NUV did not induce any activity capable of repairing psoralen cross-links in infecting lambda phage. A possible model involving removal of the cross-link by glycosylase action at the replication fork is proposed.