Regulation of horizontal gene transfer by intercellular peptide signaling in Bacillus subtilis

Abstract

Horizontal gene transfer plays an important role in bacterial evolution. Although acquisition of foreign DNA can be beneficial to cells, it can also be detrimental. Therefore, cells that possess mechanisms to regulate horizontal gene transfer likely have a competitive advantage. Similarly, several mobile genetic elements possess mechanisms that regulate transfer. This regulation maintains stability and promotes dissemination of the element, thereby ensuring its survival. Elucidating the mechanisms that regulate transfer should provide insights into conditions that favor horizontal gene transfer and bacterial evolution. This thesis describes the regulation of two means of horizontal gene transfer in the gram-positive bacterium Bacillus subtilis. Under certain conditions, B. subtilis cells undergo differentiation into competent cells capable of acquiring foreign DNA from the environment. A variety of factors regulate competence development. Initiation of genetic competence is controlled by a transcription factor, ComA, that also activates expression of genes that encode degradative enzymes, antibiotics, and secreted products important for biofilm formation.

(cont.) Three signaling peptides were known to stimulate the activity of ComA. I have characterized a fourth signaling peptide that stimulates the activity of ConmA and have shown that intercellular peptide signaling modulates the timing and levels of ComA-dependent gene expression in response to different cellular cues. B. subtilis cells also contain a mobile genetic element known as ICEBsl (integrative and conjugative element B. subtilis #1). ICEBsl is normally integrated in the chromosome of the host cell. Under certain conditions, ICEBsl excises from the chromosome and transfers through a self-encoded conjugative apparatus to recipient cells. Both the global DNA damage response and intercellular peptide signaling regulate excision and transfer of ICEBsl. The global DNA damage response stimulates ICEBsl excision and transfer and likely provides a mechanism for the ICEBsl element to escape a distressed host cell for a more suitable host. Intercellular peptide signaling limits excision and transfer of ICEBsl to conditions when successful dissemination to cells lacking ICEBsl is most likely to occur. The ICEBsl-encoded proteins that regulate excision and transfer in response to intercellular peptide signaling and the global DNA damage response are encoded by other mobile genetic elements, indicating that this may be a conserved mechanism regulating their dissemination.