Effect of PAT1 introns 1 and 2 on mRNA accumulation and transcription rate. A, RNA gel blot hybridized with GUS and a PAT1 cDNA fragment spanning exons 4 through 9. Each lane contains RNA from an independent single-copy transgenic line containing the PAT1:GUS fusion indicated. B, Nuclear run-on transcription assay. Radiolabeled RNA synthesized by nuclei isolated from the lines indicated along the top (pAR252 line A8, pAR253 line B9, and pAR254 line B4) was hybridized to filter strips on which the DNAs listed along the left had been affixed by dot blot. The relative GUS transcription rate shown is based on the radioactivity hybridized to the GUS DNA, relative to that in the pAR208 line, corrected for variations in RNA probe intensity using the PAT1 signal. The pBI121 positive control line contains the GUS gene driven by the strong cauliflower mosaic virus 35S promoter.