Superhelical DNA in Streptococcus sanguis: role in recombination in vivo.

Competent Streptococcus sanguis treated with non-lethal doses of coumermycin A1 immediately before or after uptake of radioactive transforming DNA were reduced in their capacity to yield transformants. This treatment did not alter bacterial ability to bind DNA in DNase I-resistant form, nor did it prevent the single-stranded donor DNA-recipient protein complexes formed upon uptake at the surface of the bacteria from translocating to chromosomal sites. Inhibition of transformation by heterospecific DNA was greater than that by homospecific DNA. The reduction in transformant yield was not accompanied by any loss of donor counts incorporated into the recipient chromosome, but rather by a loss of genetic activity of incorporated donor material indicating a failure of genetic integration and degradation of donor DNA as a consequence of coumermycin treatment. The inhibitory effect of coumermycin on transformation was associated with in vivo loss of chromosomal DNA superhelicity, The chromosomal DNA remained intact, however, indicative of inhibition of a gyrase-like enzyme responsible for the maintenance of negative supercoiling of the S. sanguis chromosome. Upon treatment with the drug, a coumermycin-resistant mutant strain showed neither loss of chromosomal superhelicity nor any inhibitory effect on genetic integration of donor DNA. The evidence supports the idea that chromosomal superhelicity promotes genetic recombination in vivo.