A STUDY ON THE FUNCTION OF THE GLYCINE RESIDUE IN THE YGDD MOTIF OF THE RNA-DEPENDENT RNA-POLYMERASE BETA-SUBUNIT FROM RNA COLIPHAGE Q-BETA

Q beta replicases in which the Gly residue of the beta-subunit in the motif sequence, YGDD, was replaced with Ala, Ser, Pro, Met, or Val lost their replicase activity in vivo. In an in vitro Mg2+-dependent RNA-synthesizing system using poly(rC) or MDV-poly(+) RNA (a derivative of the naturally occurring small RNA that accumulates in the cells during Q beta phage infection) as templates, the lysates from the cells expressing such defective replicases exhibited only 2-6% of the enzyme activity of the lysate from those expressing wild-type replicase. However, the defective replicases, especially A357, with Ala substituted for the Gly, recovered enzyme activity when Mn2+ was added to the reaction mixture. Furthermore, the characteristics of the MDV-poly(+) RNA-dependent RNA synthesis by A357 replicase were similar to those by wild-type replicase in the presence of Mn2+. Gel retardation assay showed that all of the defective replicases could bind MDV-poly(S) RNA. These results suggest that the Gly residue in this motif of Q beta replicase is involved in Mg2+-catalyzed polymerization. In the Mn2+-catalyzed polymerization, A357 and S357 replicases can act as well as the wild-type replicase.