| Integrative and conjugative elements (ICEs) comprise a large and diverse group of mobile genetic elements found in both Gram-positive and --negative bacteria. These elements primarily reside in a host chromosome yet retain the ability to excise and transfer via conjugation. While ICEs utilize a variety of mechanisms to promote their core functions of integration/excision, transfer and regulation, there are common features that unify this sizeable group of mobile genetic elements. The core functions of each ICE are typically encoded by modular units, with functionally related genes grouped together. Recombination among ICEs or between ICEs and other mobile elements such as phages or plasmids can promote rapid evolution through the exchange or addition/deletion of functional modules. The presence of a variety of accessory genes, such as those mediating antibiotic resistance, is also common; such genes can give each ICE and its host distinct properties. In addition, ICEs can facilitate horizontal transmission of unrelated sequences, via mobilization in trans, formation of cointegrate molecules, and Hfr-like transfer of chromosomal DNA. This thesis describes a comparative and functional analysis of the SXT/R391 family of ICEs that highlights their modular core structure and evolution. Additionally, two genes now designated mosA and mosT (for maintenance of SXT Antitoxin and Toxin), were identified that promote SXT maintenance in cell populations even in the absence of selection. Lastly, the dynamics of SXT integration and excision are explored using a single-cell microscopy approach. |
