The main early and late promoters of Bacillus subtilis phage phi 29 form unstable open complexes with sigma A-RNA polymerase that are stabilized by DNA supercoiling.

Most Escherichia coli promoters studied so far form stable open complexes with sigma 70-RNA polymerase which have relatively long half-lives and, therefore, are resistant to a competitor challenge. A few exceptions are nevertheless known. The analysis of a number of promoters in Bacillus subtilis has suggested that the instability of open complexes formed by the vegetative sigma A-RNA polymerase may be a more general phenomenon than in Escherichia coli. We show that the main early and late promoters from the Bacillus subtilis phage phi 29 form unstable open complexes that are stabilized either by the formation of the first phosphodiester bond between the initiating nucleoside triphosphates or by DNA supercoiling. The functional characteristics of these two strong promoters suggest that they are not optimized for a tight and stable RNA polymerase binding. Their high activity is probably the consequence of the efficiency of further steps leading to the formation of an elongation complex.