ROLE OF CALF RTH-1 NUCLEASE IN REMOVAL OF 5'-RIBONUCLEOTIDES DURING OKAZAKI FRAGMENT PROCESSING

The role of the exonucleolytic activity of the calf 5' to 3' exo/endon uclease, a RAD:! homolog 1 (RTH-1) class nuclease, in lagging-strand D NA replication has been examined using model Okazaki fragment substrat es. These substrates exemplify the situation in Okazaki fragment proce ssing which occurs after the initiator RNA primer is cleaved off, and released intact, by calf RNase HI, leaving a single ribonucleotide at the 5' end of the RNA-DNA junction. This final RNA is then removed by the calf RTH-1 nuclease [Turchi et al. (1994) Proc. Natl. Acad. Sci. U .S.A. 91, 9803-9807]. The cleavage specificity of calf RTH-1 nuclease for different junction ribonucleotides was compared. These were remove d without the usual requirement of calf RTH-1 for an immediately adjac ent upstream primer. In most cases, the presence of an upstream DNA or RNA primer, separated from the monoribonucleotide-DNA segment by eith er a nick or a gap, reduced the efficiency of removal of the monoribon ucleotide compared to the removal seen with no upstream primer. Substr ates in which the monoribonucleotide-DNA segment had been replaced by an oligomer of the same sequence but consisting entirely of DNA also e xhibited upstream primer inhibition. Results with various sequences in dicated that the upstream primer is generally inhibitory for ribonucle otide removal but is sometimes neutral. For deoxynucleotide removal it could be stimulatory, neutral, or inhibitory. Possible reasons for th e unexpected lack of upstream primer dependence have been explored. Th e ratio of RNase HI to RTH-1 was also shown to be critical for both en zymes to work together efficiently. These results suggest that regions of upstream primer inhibition within the genome may play a role in de termining the mechanism by which mammalian Okazaki fragments are proce ssed.