Book of Abstracts: Albany 2003
June 17-21 2003
A Multicomponent, Multifunctional Intron Endonuclease, I-TevI
I-TevI is a homing endonuclease that is encoded by and promotes the mobility of the td intron of phage T4. This 28-kD protein is remarkable in that it is site specific, yet recognizes a target site of ~37 bp in a sequence-tolerant fashion. The enzyme consists of two functionally distinct domains, an N-terminal catalytic domain and a C-terminal DNA-binding domain connected by a 75-amino acid linker. What has previously been defined as the DNA-binding domain has an extended structure consisting of three distinct modules which contact ~20 bp of the homing site: a Zn finger, an α-helix and a small helix-turn-helix subdomain (1). It is now known that the zinc finger is not required for DNA binding or catalysis, but rather is a spring-like component of the linker that directs the catalytic domain to cleave the homing site at a fixed distance from the intron insertion site (2). In contrast, the catalytic domain, which contains the conserved GIY-YIG motif characteristic of this family of homing endonucleases, is a compactly folded structure. It is provocative that there are similarities in the three-dimensional arrangement of the catalytically important residues and the cation-binding site with those of I-PpoI, a member of the His-Cys-box family of homing endonucleases (3). These results suggest the possibility of mechanistic relationships among these different families of enzymes, but whether this represents convergent or divergent evolution is not known. Regardless, the picture that has built is one of a catalytic GIY-YIG cartridge that has become associated with modules that direct DNA binding and distance measurement. Surprisingly, the DNA-binding domain has recently been shown to interact with regulatory sequences upstream of the I-TevI gene, such that I-TevI acts as a repressor of its own synthesis. Thus, I-TevI is a multicomponent, multifunctional molecule that has evolved site-specific cleavage and autorepression functions to maximize the invasiveness and persistence of its host intron.