Primer extension to map the crosslink sites in pTR14. Lanes G, A, T, and C represent dideoxynucleotide-terminated sequencing reactions by using unmodified pTR14 RNA as template. Lane 1, pTR14 crosslinked to holoenzyme reconstituted with AzP-S49C P protein; lane 2, pTR14 crosslinked to holoenzyme reconstituted with AzP-V26C P protein; and lane 3, uncrosslinked pTR14. The primer extension products are separated on an 8% polyacrylamide-urea gel. The corresponding nucleotide sequence of the pre-tRNA leader is shown. NT and CS represent the nucleotides in the cleft and cleavage site, respectively.