The behavior of X phage in the Rec-strain JC-1569 is compared with that in the Rec+ strain JC-1557. No difference deemed significant was noted in the adsorption rate, latent period, burst size, frequency of lysogenization, and frequency of vegetative phage recombination. The location of the prophage and its mode of insertion in the Rec-lysogen of wild-type X (X+) were inferred to be normal from the results of conjugational crosses. Spontaneous and ultraviolet (UV) irradiation induction of X+ were markedly reduced in the Rec-lysogen. On the other hand, thermal induction of a mutant lambda (Xc1857) lysogen of the Rec-strain was not reduced and was only slightly affected by UV irradiation. Phage subject to inhibition by X immunity failed to multiply in UV-irradiated cells of the Rec-X+ lysogen, whereas those not inhibited by this immunity did multiply. It was concluded that the failure of UV to induce X+ in the Rec-lysogen was not due to damage to the prophage, but rather to the inability of the irradiated cells to respond by lifting immunity. Preliminary evidence indicates that a single mutation confers recombination deficiency and the inability to lift immunity after UV irradiation. Possible relationships between recombination and the lifting of immunity are enumerated.
A recombinant plasmid, designated pUC1002, was constructed by ligation of a HindIlI restriction endonuclease fragment of Escherichia coli chromosomal DNA to vector plasmid pMB9. Strains carrying this plasmid were selected by transformation of an E. coli strain bearing the xyl-7 mutation to a xylose-positive (Xyl+) phenotype. Strains containing pUC1002 produced coordinately elevated levels of D-xylose isomerase and D-xylulose kinase. Under appropriate conditions, the isomerase also efficiently catalyzed the conversion of D-glucose to D-fructose. A number of bacteria, including Escherichia coli, contain D-xylose isomerases which function physiologically to catalyze the first reaction in the catabolism of D-xylose but which also convert D-glucose to D-fructose under certain conditions (24). The latter reaction is the basis for industrial processes currently used to produce very large quantities of high-fructose syrups (4). This use, as well as interest in the use of the enzyme to promote efficient fermentation of D-xylose to ethanol (5, 12, 21, 22; T. W. Jeffries and S. Choi, Abstr. Annu. Meet. Am. Soc. Microbiol. 1981, 034, p. 193), led us to try to increase the yield of the enzyme by gene cloning. This report describes the construction and selection of a recombinant plasmid coding for both E. coli D-xylose isomerase and E. coli Dxylulose kinase, the second enzyme in the pathway for D-xylose utilization by E. coli. We also describe the production of the enzymes and the D-glucose isomerase activity in strains bearing the plasmid. The recipient strain used in the cloning was JC1553 (6), an E. coli K-12 derivative obtained from the E.
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