is a characteristic of many viral capping systems this type of modular extension of sam rna binding domain might be found in other viruses for which guanylyltransferases have yet to be identified 6 there are a number of original structural features of cef that differ from known nucleotide binding site structures exemplified by the contacts made by the purine base the guanine specificity is achieved via three specific interactions of main chain carbonyl groups with the 2 amino group of guanine as shown in fig3 b none of these interactions would be possible with adenine thus the specificity for guanine vs adenine binding does not involve specific interactions of the protein with the c6 purine substituent this type of nucleotide discrimination appears to be novel the n7 position of guanine points towards the solvent and does not contact any residue this is different from the vp39 enzyme for which the alkylated base is a determinant of binding specificity 19 ribavirin 5 u2032 triphosphate binds to cef under the same conditions as those used for the gtp analogue and makes the same contacts as the gtp analogue as shown in fig4 a interestingly the carbonyl group of ribavirin does not interact with any residue but the nh 2 group hydrogen bonds the same carbonyl groups of leu 17 asn 18 and leu 20 as the nh 2 of the gtp analogue compare fig3.