Cloning of the genetopA encoding for DNA topoisomerase 1 and the physical mapping of thecysB–topA–trpAregion ofEscherichia colt

Abstract

The gene top A of Escherichia coli that encodes for DNA topoisomerase I has been cloned by a combination of genetic and radioimmunal screening. The gene has been mapped to be within a 3.4 Kb segment of the bacterial genome. The intracellular level of the enzyme in strains harboring extra chromosomal copies of top A gene increases with increasing copy number of the gene and the introduction of extra chromosomal copies of the top A gene truncated at its 3’ side into a top A strain of E. coli does not significantly influence the expression of the chromosomal copy of top A. These results suggest that the expression of top A is not tightly regulated. Strains in which DNA topoisomerase I is overproduced grow significantly slower in broth and give smaller size colonies on agar plates. Physical mapping of a 20 Kb region containing cysB; top A and trp A has also been carried out with a number of restriction enzymes; top A is found to be immediately adjacent to cys B and is separated from trp A by a 7 Kb segment where no known gene resides.