Mapping construct RNA initiation sites. (A) S1 protections showing comparable amounts of transcripts correctly initiated at the human β-actin promoter start site (nt +1 corresponding to a protection of 975 nt). A few minor start sites upstream of the promoter and in the IVS-I are flushed on in theβAPr-Ig cell line. The 1.4-kb human β-actin promoter fragment (34) in bluescript (p229.6, see the Materials and Methods) was end-labeled at the EcoRI site and used for S1-protection of the transfected human allele. (B) The primer extension assay verifies the location of additional intron start sites in the βAPr-Ig cell lines, which are also seen as the middle bands on the S1 protection in Fig 5 (βAPr-Ig). A 20-nt primer starting from theEcoRI site of the β-APr plasmid (p229.6) was used.