Figure 2.

The ribozyme-connected DHFR expression vector. The plasmid vector has two ATG codons, one of which is a pseudo-initiation codon, located upstream of the authentic ATG, which is the initiation codon for the DHFR gene. If an active ribozyme is introduced upstream of the DHFR-coding region and if, upon transcription, the primary transcript is cleaved by this cis-acting ribozyme at the predetermined site between the two ATG codons, the excised mRNA can produce DHFR. Otherwise, the primary transcript starts translation at the pseudo-initiation codon, which is associated with a strong Shine–Dalgarno (SD) sequence and is out of frame with respect to the DHFR gene.