The prototypes of Seg family are proteins SegA, B, C, D, E, F, and G encoded by five seg genes segA, B, C, D, E, F, and G in the bacteriophage T4 genome, respectively. SegA, B, C, D, E, F, and G are not encoded by introns, but free-standing homologs of the GIY-YIG family of endonucleases encoded by group I introns, which are thought to initiate the homing of their own intron by cleaving the intronless DNA at or near the site of insertion. Both phage T4 intron-encoded and free-standing GIY-YIG endonucleases contribute to the exclusion of T2 markers from the progeny of mixed infections. SegA, encoded by the bacteriophage T4 segA gene, is a double-strand DNA endonulcease with a hierarchy of site specificity. The cleavage site of SegA is located in the uvsX gene of T4. Its cleaving activity requires the presence of Mg2+ and can be stimulated by the presence of ATP or ATPgammaS. Bacteriophage T4 segB gene encoding protein SegB is a site-specific endonuclease that recognizes a 27-bp sequence, cleaves DNA by introdu cing double-strand breaks in the adjacent gene 56 of T2 during mixed infection in the presence of Mg2+, Mn2+, or Ca2+ cations, and produces mostly 3' 2-nt protruding ends at its DNA cleavage site. It functions as a homing endonuclease to ensure spreading of its own gene and the surrounding tRNA genes among T4-related phages. Bacteriophage T4 segE gene encoding SegE is a site-specific endonuclease that preferentially cleaves DNA in a site located at the 5' end of the uvsW gene in the RB30 genome. It is responsible for a non-reciprocal genetic exchange between T-even-related phages. Bacteriophage T4 gene 69 encoding SegF is a site-specific double-strand DNA endonuclease that promotes marker exclusion. It preferentially introduces a double-strand break in the adjacent T2 gene 56 over T4 gene 56 both in vitro and in vivo during mixed infection, which results in the replacement of T2 gene 56 by T4 gene 56 in a process similar to group I intron homing. The cleavage site is located 210- and 212-bp upstream from its insertion site. Bacteriophage T4 segG gene (formerly gene 32.1) encoding SegG (also known as F-TevIV) is a double-strand DNA endonuclease adjacent to gene 32 of phage T4 that promotes marker exclusion. Although it is absent from phage T2, SegG preferentially introduces a double-strand break in T2 gene 32 during mixed infection, which results in replacement of T2 genetic markers by the corresponding T4 markers. The cleavage site is located 332- and 334-bp from its insertion site.
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