The SXT/R391 Family of Integrative Conjugative Elements Is Composed of Two Exclusion Groups

Abstract

Conjugative elements often encode entry exclusion systems that convert host cells into poor recipients for identical or similar elements. The diversity of exclusion systems within families of conjugative elements has received little attention. We report here the most comprehensive study to date of the diversity of exclusion determinants within a single family of conjugative elements. Unexpectedly, our analyses indicate that there are only two exclusion groups among the diverse members of the SXT/R391 family of integrative conjugative elements.Integrative and conjugative elements (ICEs) are a diverse group of mobile genetic elements with the capacity to incorporate and disseminate genes encoding a variety of properties, including antibiotic resistance, into many bacterial hosts (6). ICEs are self-transmissible, and they transfer from cell to cell via conjugation, similar to conjugative plasmids. However, unlike conjugative plasmids, ICEs do not replicate autonomously; instead, they integrate into the host chromosome. SXT and R391 are closely related ICEs derived from clinical isolates of Vibrio cholerae and Providencia rettgeri, respectively (9, 23). SXT and R391 were the first recognized members of the SXT/ R391 family of ICEs, which now includes ϳ25 members (5). These ICEs were grouped as a family since they encode nearly identical integrases that mediate the integration of the elements into the host chromosome at the prfC locus (8,14). In addition, the members of the SXT/R391 family of ICEs appear to carry a conserved set of genes involved in DNA transfer (8).The conjugal transfer genes in SXT and R391 are distantly related to those found in the F plasmid (4). Similar to F (3) and other conjugative plasmids (11), SXT and R391 carry genes for an entry exclusion system mediated by two inner membrane proteins, one (TraG) in the donor and the other (Eex) in the recipient (16). Entry exclusion systems function to specifically inhibit redundant conjugative transfers between cells that carry the same element (1,10,11,21). We recently showed that even though SXT and R391 have nearly identical conjugative transfer genes, these ICEs do not exclude each other (16). Thus, cells harboring SXT inhibit the acquisition of SXT but not R391 and vice versa. We found that element-specific exclusion activity is mediated by Eex variants EexS and EexR and TraG variants TraG S and TraG R (encoded within SXT and R391, respectively). For example, exclusion was observed when EexS was produced in the recipient and TraG S in the donor; however, EexS did not exclude ICE transfer from a donor producing TraG R . The specificity of the Eex variants is dictated by residues found in their respective carboxyl termini. While the first 86 amino-terminal residues of EexS and EexR are 87% identical, the remaining 56 carboxyl-terminal residues are only 41% identical. Interestingly, a stretch of only 3 amino acids (amino acids 606, 607, and 608) determines TraG's exclusion specificity. At these positions, TraG S has the amino acid sequence P-G-E, wher...

Biology of Three ICE Families: SXT/R391, ICE Bs1 , and ICE St1 /ICE St3

“…It excludes itself and all other SXT/R391 ICEs bearing an eexR allele but not those bearing an eexS allele. Remarkably, every ICE bearing an S group eex gene also bears an S group traG gene and similarly for ICEs of the R group (55). The comparison of the variants of traG/eex pairs and the construction of chimeric mutants allowed the specificity of exclusion in TraG to be pinpointed to only three amino acids at position 606-608, TDD or TGD for TraG R and PGE for TraG S (53,55).…”

“…Remarkably, every ICE bearing an S group eex gene also bears an S group traG gene and similarly for ICEs of the R group (55). The comparison of the variants of traG/eex pairs and the construction of chimeric mutants allowed the specificity of exclusion in TraG to be pinpointed to only three amino acids at position 606-608, TDD or TGD for TraG R and PGE for TraG S (53,55). In the 143 amino acids Eex protein, specificity determination was attributed to the last 56 C-terminal residues which exhibit the highest divergence (41% identity) (55).…”

“…All members of the SXT/R391 family do not exclude each other. In fact, two variants of the exclusion system coexist within the family: the S and R groups (55). SXT is the prototypical member of the S group and excludes the entry of all other SXT/R391 ICEs bearing an eexS allele but not those bearing an eexR allele.…”

“…The comparison of the variants of traG/eex pairs and the construction of chimeric mutants allowed the specificity of exclusion in TraG to be pinpointed to only three amino acids at position 606-608, TDD or TGD for TraG R and PGE for TraG S (53,55). In the 143 amino acids Eex protein, specificity determination was attributed to the last 56 C-terminal residues which exhibit the highest divergence (41% identity) (55). The reasons for the emergence and persistence of two exclusion groups in a single family of ICEs are unknown.…”

Comparative Analysis of Mobilizable Genomic Islands

“…However, RecA could be responsible for MGI recombination, and the involvement of the ICE-encoded recombination system is also likely. In the future, it would be interesting to study the ability of MGIs to form tandems and verify whether some MGIs exclude others, as it was reported for the SXT/R391 ICEs (45,46).…”