The expression of the DNA ligase gene of Escherichia coli is stimulated by relaxation of chromosomal supercoiling.

Many genes of Escherichia coli have been shown to be sensitive to DNA superhelicity. The superhelicity of the chromosome is itself also supercoiling-dependent. We have developed a general strategy for investigating how a particular gene responds to changes in DNA topology. This approach is used to study the E. coli ligase gene. The thermosensitivity of the E. coli ligts251 mutation can be phenotypically suppressed by mutations which map close to, or in, the gyrB gene and which affect the degree of DNA supercoiling. The level of suppression correlates with the degree of DNA relaxation observed, suggesting that the gene encoding the E. coli DNA ligase is activated by relaxation of the chromosomal DNA.