Modified aptamer-expression cassettes with 5′-flanking and 3′-flanking extended hammerhead ribozymes. (a) Oligonucleotides corresponding to the intended sequences were annealed, PCR amplified, digested with NheI and NotI restriction enzymes, and ligated in place of the mcHHRz aptamer-expression cassette. Individual aptamers were cloned into modified cassettes using the EcoRI and BamHI restriction sites. Modified extended hammerhead ribozymes (eHHRz) cassettes include inactive or highly active RzB (5′) and inactive or highly active satellite tobacco ring spot virus (sTRSV) (3′). Active eHHRz contain a CUG within the catalytic core, indicated by a solid black arc. The CUG was mutated to GAA in the inactive variants of the eHHRz. (b) In vitro cotranscriptional cleavage activity of the mcHHRz and eHHRz aptamer-expression cassettes expressing aptamer 70.05. Expected cleavage products and locations are illustrated by the boxes on the right, with lines directed toward the expected products. Product sizes are given in Supplementary Table S3. The aptamer-containing fragment is denoted by an open triangle.