TABLE 1.

Classifications of T4SSs.

Criterion	Sub-families	Reference
Pilus type	F-like, P-type, I-type	Lawley et al., 2003
Relaxase phylogeny	MOB(F), MOB(H), MOB(Q), MOB(C), MOB(P), MOB(V)	Garcillán-Barcia et al., 2009
Substrate	Relaxase and ssDNA (conjugative systems), effector translocator systems, DNA release and uptake systems	Alvarez-Martinez and Christie, 2009
VirB4 phylogeny	MPFI, MPFC, MPFG, MPFT, MPFF, MPFB, MPFFATA, MPFFA	Guglielmini et al., 2013
Function	cT4SS, pT4SS	de Paz et al., 2005
Protein components	T4ASS, T4BSS	Christie et al., 2017; Grohmann et al., 2018