Mutations in the SRV-1 frameshifting signal were made in plasmid SF2 that is derivative of pSFCASS5 (8), a frameshift reporter construct used in earlier phenotypic studies. SF105-113 [except SF110 (A28G) see SF210] were obtained by a two-step PCR mutagenesis procedure using degenerated primers on plasmid pSF103 that contains the sequence of the 'NMR pseudoknot' (26) . The appropriate mutants were selected by dideoxy sequencing.