SELF-SPLICING GROUP-I AND GROUP-II INTRONS ENCODE HOMOLOGOUS (PUTATIVE) DNA ENDONUCLEASES OF NEW FAMILY

new family of protein domains consisting of 50-80 amino acid residues is described. It is composed of nearly 40 members, including domains encoded by plastid and phage group I introns; mitochondrial, plastid, and bacterial group II introns; eubacterial genomes and plasmids; and phages. The name ''EX(1)HH-HX(3)H'' was coined for both domain and family. It is based on 2 most prominent amino acid sequence motifs, each encompassing a pair of highly conserved histidine residues in a specific arrangement: EX(1)HH and HX(3)H. The ''His'' motifs often alternate with amino- and carboxy-terminal motifs of a new type of Zn-finger-like structure CX(2,4)CX(29-54)[CH]X(2,3)[CH]. The EX(1)HH-HX(3)H domain in eubacterial E2type bacteriocins and in phage RB3 (wild variant of phage T4) product of the nrdB group I intron was reported to be essential for DNA endonuclease activity of these proteins. In other proteins, the EX(1)HH-HX(3)H domain is hypothesized to possess DNase activity as well. Presumably, this activity promotes movement (rearrangement) of group I and group II introns encoding the EX,HH-HX,H domain and other gene targets. In the case of Escherichia coli restrictase McrA and possibly several related proteins, it appears to mediate the restriction of alien DNA molecules.