Figure 3.

The RNA·DNA hybrid extends to the promoter proximal end of the repeat tract. Pairs of transcription reactions end-labeled by including gamma ³²P-GTP, were done in the absence (−) or presence (+) of RNase H to map the 5′ end of the RNA hybrid. Lanes 1–6 contain the end-labeled products derived from supercoiled templates with 0 (lanes 1 and 2), 11 (lanes 3 and 4) or 44 (lanes 5 and 6) GAA·TTC triplets. Lanes 7 and 8 contain the end-labeled products derived from a linear (GAA·TTC)₄₄ template. The most common end-labeled fragments generated by RNase H were at or near the start of the GAA tract in the templates with 44 triplets (lanes 6 and 12). Lane 13 contains a ‘G ladder’ from a partial RNase T1 digest of end-labeled (GAA)₄₄ transcript. The 3′ end of the (GAA)₄₄ tract at base 187 is indicated by a thin arrow, the first G of the triplet repeat sequence, at base 55 in the transcript 5′ end is indicated by a thick arrow near the bottom of the gel. Full-length transcripts are not resolved on this gel (but see Figure 1).