A specialized Mu transducing phage containing a gene encoding ampicillin resistance and the lac structural genes without the lac promoter [Mu d(Apr lac)] has been constructed and used to create gene fusions in Escherichia coli (M. J. Casadaban and S. N. Cohen, Proc. Natl. Acad. Sci. U.S.A. 76:4530-4533, 1979). Transposition of the Mu d(Apr lac) phage to chromosomal sites can result in lac expression being controlled by a chromosomal promoter. We have constructed an Escherichia coli K-12 strain in which the Mu d(Apr lac) phage is integrated into an F factor. The F+::Mu d(Apr lac) was then transferred by conjugation into a Salmonella typhimurium strain that was sensitive to L-arabinose. Strains containing gene fusions were selected as L-arabinose-resistant colonies after partial induction of the phage. Two classes of ara-lac fusion strains were isolated: (i) araC-lac fusions in which the expression of,-galactosidase synthesis was constitutive and not inducible by L-arabinose; and (ii) fusion of the lac genes to the ara structural genes in which the expression of fl-galactosidase synthesis was induced 263-fold by L-arabinose.
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