Initiator RNA synthesis upon ribonucleotide depletion. Evidence for base substitutions

Nearest neighbor frequencies in initiator RNA (iRNA) of a human lymphocyte cell line have been measured in isolated nuclei under conditions where ribonucleotides have been omitted from the incubation. Deoxynucleotides are incorporated in iRNA to a limited extent for the omitted cognate ribonucleotide. In addition, upon omission of a ribonucleotide, changes in the nearest neighbor frequencies also occur indicating base substitution of another ribonucleotide. The base substitutions that have been observed are the incorporation of rA for rG or rU. Substitution for rA has not been observed. When rC is omitted, a change in all nearest neighbor frequencies occurs, suggesting possibly an unusual role for rC. The substitutions demonstrate the unique features of iRNA synthesis and point out possible alterations in iRNA that may occur in vivo when nucleotide pools are altered.