3-Galactoside Transport in a Proteus mirabilis Merodiploid Carrying an Escherichia coli Lactose Operon

Merodiploid derivatives bearing an F-linked lac operon (i+, o+, z+, v+, a+) from Escherichia coli were prepared from a Proteus mirabilis strain unable to utilize lactose and from a lac deletion strain of E. coli. A suitable growth medium was found in which the episomal element in the P. mirabilis derivative was sufficiently stable to allow induction of the episome-borne lac operon and thus to permit a comparison of the activities and properties of E. coli lac products in the intracellular environments of P. mirabilis and E. coli. In both derivatives the episomal lac operon was shown to be repressed in the absence of inducer. Kinetics of induction with gratuitous inducer (isopropyl-1-thio-B-D-galactoside) were similar for both ,-galactosidase activity (,B-D-galactoside galactohydrolase, EC 3.4.1.23) and ,-galactoside transport activity in both derivatives, although the ratio of galactoside transport to ,-galactosidase activity was approximately 1.6-fold higher in the E. coli derivative. Comparison of f-galactosidase and M-protein (lac y gene product)-specific activities indicated coordinate expression of the induced lac operon in both derivatives. Quantitatively, the maximal Il-galactosidase specific activity was two or three times higher for the E. coli derivative. A significant sodium azide inhibition (65%. inhibition by 10 mM sodium azide) of lactose permease-mediated transport of o-nitrophenyl-,B-galactoside from an outside region of high concentration to an inside region of very low concentration ("downhill transport") was observed for the P. mirabilis derivative. Identical conditions for the E. coli derivative yielded only about 15% inhibition. Active transport of thiomethyl-I8-galactoside was similar for both derivatives, the major difference being that active transport was more sensitive to azide poisoning in the P. mirabilis derivative. Preliminary examination of the thiomethyl-,B-galactoside derivatives following active transport did not demonstrate the accumulation of a phosphorylated product in either strain but did reveal an unidentified derivative present in the P. mirabilis merodiploid extract which was not detectable in the E. coli merodiploid.