Constructs were produced by amplification of complementary oligonucleotides (Integrated DNA Technologies) to produce a full-length sequence containing 5′ Spe I and 3′ Bam HI restriction sites. These were cloned into the vector pJ307 (GST-MBP-His fusion vector) (30 (link)). To distinguish between the termination product and the trans-frame product, we present the MBP tag in the alternative −1 frame relative to the GST tag. MBP and GST were in frame for the positive control. Primers used in this study are provided separately in the Supplementary Materials.