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A poliovirus replicon, FLC/REP, which incorporates the reporter gene chloramphenicol acetyltransferase (CAT) in place of the region encoding the capsid proteins VP4, VP2, and part of VP3 in the genome of poliovirus type 3, has been constructed. Transfection of cells indicates that the FLC/REP replicon replicates efficiently and that active CAT enzyme is produced as a CAT-VP3 fusion protein. The level of CAT activity in transfected cells broadly reflects the level of FLC/REP RNA. A series of mutations in the 5' noncoding region of poliovirus type 3 were introduced into FLC/REP, and their effects were monitored by a simple CAT assay. These experiments helped to define further the stem-loop structures in the 5' noncoding region which are essential for RNA replication. The CAT-containing poliovirus replicon could also be packaged into poliovirus capsids provided by helper virus and was stable as a subpopulation of virus particles over at least four passages. The location of the CAT gene in FLC/REP excluded the presence of an encapsidation signal in the region of the poliovirus genome comprising nucleotides 756 to 1805.