RNA LIGASE which was discovered by Silber et al.1 in T4-infected Escherichia coli cells, catalyses an ATP-dependent, intramolecular joining reacion of 5′-P and 3′-OH termini of various homopolyribonucleotides1,2. The shortest circularising polyadenylate was shown to be (pA)8, the optimal chain length for the reaction being 10-30 (ref. 2). These facts suggest that the substrate for T4 RNA ligase is a pair of suitably juxtaposed, 3′-OH and 5′-P termini, each a few single stranded residues long2.
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