Expression of conjugation genes is controlled by a novel zipper-type transcriptional attenuation mechanism

Proper expression of genes clustered in operons, particularly large operons, can be complex, often involving multiple regulatory switches. The conjugation operon present on the Bacillus subtilis conjugative plasmid pLS20 is over 32 kb long. This operon starts with a 456-nt leader region, which is followed by the first two genes of the operon, encoding a two-component processive antitermination system. Here, we demonstrate that the long leader region encodes a transcriptional attenuator that we named cATTpLS20. In vivo and in vitro analyses showed that the attenuator is composed of three segments: a stem-loop structure with a long imperfect stem that is preceded by a sequence that may form a weak stem-loop and followed by a sequence that may form an intrinsic terminator. Sequences of the upstream stem-loop and downstream terminator are complementary, allowing further extension of the long stem, thereby generating an antiterminator conformation. Based on similarity with a zipper, we coined this the zipper-type attenuator. Similar zipper-type attenuators are present upstream of the first genes of conjugation operons of all pLS20 family plasmids, as well as on many other conjugative plasmids in Gram-positive bacteria, suggesting that a common attenuation mechanism regulates expression of many conjugative operons.