EVOLUTION OF HOST-CELL RNA INTO EFFICIENT TEMPLATE RNA BY Q-BETA REPLICASE - THE ORIGIN OF RNA IN UNTEMPLATED REACTIONS

Q beta replicase can replicate a single molecule of certain species of RNA to 10(14) copies in minutes. This replication ability has been us ed for in vitro studies of molecular evolution and is currently being utilized as a method of amplifying RNAs that contain probe sequences. It has been observed that Q beta replicase can produce replicatable RN A even in the absence of exogenously added template RNA. The origin of this RNA has been ascribed either to contamination with replicatable RNA or to an ability of Q beta replicase to synthesize RNA de novo fro m the nucleotides present in the reaction. Technologies that employ Q beta replicase require a thorough understanding of the conditions that lead to this so-called spontaneous RNA production. We have created an expression system and purification method with which we produce gram quantities of highly purified Q beta replicase, and we have identified reaction conditions that prevent the amplification of RNA in assays t hat do not contain added RNA, However, when these reaction conditions are relaxed, spontaneous RNA replication is seen in up to 100% of the assays. To understand the origin of this RNA, we have cloned several s pontaneously produced RNAs. Sequence analysis of one of these RNAs sho ws that it arose by the evolution of Escherichia coli tRNA into a repl icatable template and not by de novo synthesis from nucleoside triphos phates in the reaction.