The genes of the Salmonella histidine operon (his) have been placed on an F′ pro lac plasmid using genetic methods that rely on recombinational homology provided by Tn10 transposon insertions. The position and orientation of the transposed his genes permit subsequent deletion mutations to form operon fusions that put the lac genes under his operon control. Strains carrying such fusions show co-ordinate regulation of histidinol dehydrogenase and beta-galactosidase expression. While all of the operon fusions have an intact hisD gene, complementation testing and deletion mapping reveal that the genes downstream of hisD are deleted to varying extents. The beta-galactosidase produced by these operon fusions is itself a fused protein containing the amino terminus of one or another of the his enzymes. Two of the operon fusions having join-points in the hisB gene retain histidinol phosphate phosphatase activity and may produce a bifunctional protein having beta-galactosidase as well as the phosphatase activity. The methods that have been used to isolate these his-lac fusions should be applicable to other genetic systems.
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