The large subunits of herpes simplex virus types 1 and 2 ribonucleotide reductases contain unique aminoterminal regions comparising 311 and 318 residues respectively, which are not found in ribonucleotide reductases from other sources. We report the mapping of the epitope recognized by monoclonal antibody 1026, which is specific for the large subunit (R1) of HSV-1, and then deduce the structural relationship of the amino-terminal region of R1 with the rest of the protein. A panel of 10 fusion proteins containing sequences spanning the entire R1 subunit were constructed. They were used together with proteolytic fragments of R1 and several synthetic peptides to show that the epitope is discontinuous and appears to be a loop structure centred on a previously located trypsin-sensitive site at residue 305. The existence of the loop was suggested by the observation that reactivity of the antibody with R1 could be blocked by peptides corresponding to residues 289 to 303 and 308 to 313 which flank the trypsin-sensitive site. Our results suggest that the unique amino-terminal region of R1 consists of a structurally distinct domain which is linked to the conserved carboxy region by an exposed loop.


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