Template-directed synthesis on the oligonucleotide d(C 7-G-C 7)

Abstract

When the deoxynucleotide template d(C 7-G-C 7) is incubated with the activated nucleotides 2-MeImpG † † Abbreviation used: 2-MeImpC, cytidine-5′-phospho-2-methylimidazolide; 2-MeImpG, guanosine-5′-phospho-2-methylimidazolide; pC, cytidine-5′-monophosphoric acid; pG, guanosine-5′-monophosphoric acid; G n C, oligonucleotides with composition G n C and terminated by a 5′ phosphate, n < 14; (pG) i pC(pG) j , oligonucleotides with the sequence G i C-G j and terminated by a 5′ phosphate; HPLC, high-pressure liquid chromatography; u.v., ultraviolet light. and 2-MeImpC, a series of oligomers of G up to the sevenmer and a series of copolymers of composition G n C with n = 3 to 13 are formed. Oligomers G n C with n > 7 are completely degraded by pancreatic ribonuclease, establishing that they contain a 3′ to 5′ internucleotide bond between 5′-C and 3′-G within a sequence of the form (pG) i pC(pG) j . As expected, (pG) 7-Cp and (pG) 6-Cp are major hydrolysis products. Detailed analysis of the product distribution shows that a substantial fraction of the oligomeric products are of the type (pG) i pC(pG) j with i < 7. This shows that product synthesis does not necessarily begin at the 3′ terminus of the template. The significance of this finding in terms of the origin of molecular replication is discussed.