Abstract
The gene topA 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 extrachromosomal copies of topA gene increases with increasing copy number of the gene and the introduction of extrachromosomal copies of the topA gene truncated at its 3' side into a topA strain of E. coli does not significantly influence the expression of the chromosomal copy of topA. These results suggest that the expression of topA 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; topA and trp has also been carried out with a number of restriction enzymes; topA is found to be immediately adjacent to cysB and is separated from trp by a 7 Kb segment where no known gene resides.