X3 is an amino acid, or a polyamide linked via its carboxy terminus, NUC is a nucleoside of any one of the formulas: ##STR1## where ???indicates the bond to the --C.tbd.C-- group in formula (I), and X4 is a sugar group of the formula: ##STR2## where the 5' oxygen is linked to Nu, and X5 and X6 are each independently, H or OR, where R is H, a protecting group, or a solid phase matrix, and Nu is an