INTERACTIONS OF CALF THYMUS AND HUMAN PLACENTAL DNA-PRIMASES WITH OLIGONUCLEOTIDE TEMPLATES OF VARIOUS LENGTHS AND STRUCTURES

Michaelis constants for poly(dT)-template-directed ligation of RNA oli gonucleotide primers catalyzed by calf thymus or human placenta DNA-pr imases increased linearly with increasing length of the template up to 10 nucleotides. There was no further increase in the constant suggest ing that 10 links of the oligonucleotide template have effective conta cts with the active sire of the enzyme. This may determine the length of riboprimers; d(pT)(2) is the minimal template suitable for riboprim er synthesis. The integrity of the carbohydrate-phosphate backbone of the template is probably not essential for the synthesis of primers. O ligoriboadenylate primers of up to 10 nucleotide length are synthesize d processively. The accumulation of di- and trinucleotide products is due to dissociation of short primers from the enzyme-template complex Possible mechanisms of DNA-primer-catalyzed synthesis of riboprimers a nd the ''switch'' from primase to polymerase activity are discussed.