Evidence for independent supercoil domains in bacterial chromosomes is over 30 years old. Recent evidence shows that in both E. coli and Salmonella the mean domain size is 10 kb (>400 domains/genome equivalent) and that most domains vary in position from cell to cell (1, 2). However, strong transcription produces supercoil domains that are region specific (3). Using site specific recombination and a defined 12 kb domain (see below), we found that domain boundaries appear of within 500 bp (down stream) of a transcription stop site. Transcription-induced domains in the mass population appear and disappear within 15 minutes of induction or repression of transcription. This time lag represents a form of chromosomal bookmarking or epigenetic memory. Domain number is controlled at least in part by the activity of DNA gyrase, and the domain number must reside within specific limits to permit cell growth and DNA replication. A model for the formation of transcription induced domains is illustrated below.


References and Footnotes

1. Postow, L., Hardy, C. D., Arsuaga, J., and Cozzarelli, N. R. Genes Dev. 18, 1766-1779 (2004).
2. Stein, R., Deng, S., and Higgins, N. P. Mol. Microbiol., In Press (2005).
3. Deng, S., Stein, R. A., and Higgins, N. P. Proc. Natl. Acad. Sci. USA 101, 3398-3403 (2004).