SecReT4
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T4SS classification schemes

T4SSs are multiunit assemblages which are typically coded for loci exhibiting differing gene contents, gene orders and the number of shared homologues. Two classification schemes have been proposed to classify T4SSs based on gene organization, shared homologies and evolutionary relationship [Wallden, et al (2010) Cell Microbiol].

Christie et al [Christie, et al (2005) Annu Rev Microbiol; Christie and Vogel (2000) Trends Microbiol] classified T4SSs into type IVA and IVB, with unassigned members allocated into an ‘others’ category. Type IVA systems are composed of subunits similar in composition and number to those of the archetypal Agrobacterium tumefaciens VirB/D4 system [Christie (2004) Biochim Biophys Acta]. Type IVB systems are assembled from subunits related to the archetypal Legionella pneumophila Dot/Icm system [Segal, et al (2005) FEMS Microbiol Rev]. The “other” systems bear little or no discernible ancestral relatedness to the types IVA or IVB systems and have been further classified.

Lawley et al [Lawley, et al (2003) FEMS Microbiol Lett] proposed a distinct T4SS classification system with types F, P and I instead, based on the incompatibility groups IncF, IncP and IncI, respectively, of the plasmids that coded for the archetypal members of each of these classes. Interestingly, all type F and type P systems identified to date fall into the type IVA group using the Christie et al classification system, while all type I systems map to a type IVB category.

In addition, the newly described conjugative T4SSs that are present in a wide variety of syntenically-related genomic islands (GIs) such as ICEHin1056, SPI-7, PAPI and pKLC102, but that are poorly related to previously described systems, have been categorized as GI type T4SSs. [Juhas, et al (2007) J Bacteriol]. GI type T4SSs are evolutionarily distant from the previously described systems. The name ‘GI’ indicates this type of T4SS is to date only associated with genomic islands.


Finally, evolutionarily-distinct T4SSs that mediate the conjugative transfer of plasmids and ICEs and thus promote dissemination of multiple-antibiotic resistance and other virulence-associated traits have recently been described in an increasing number of Gram-positive bacteria [Alvarez-Martinez, et al (2009) Microbiol Mol Biol Rev; Wallden, et al (2010) Cell Microbiol; Abajy, et al (2007) J Bacteriol].

Here, we propose that GI type T4SSs and those present in Gram-positive bacteria be designated type IVC and type IVD, respectively. In another scheme, they were called type G [ Smillie, et al (2010) Microbiol Mol Biol Rev] and type GP (Gram-positive).


Classification of T4SSs (Click on the types to view members)

Classification Scheme 1 Number Classification Scheme 2 Number T4SSs
Type IVA
617
Type F
156
Tra_F
Type P
461
VirB
Trb
Trw
Ptl
Lvh
ComB
Cjp
Cag
MagB
Type IVB
37
Type I
37
Tra_I
Dot/Icm
Type IVC ( GI type)
97
Type G
97
Tfc
Type IVD ( Gram + )
60
Type GP
60
Prg/pcf
Orf_p
Ydd
Tcp
Orf_Tn