FIG. 3.

Poly(A) signals become silent when close to the 5′ end. (A) Schematics of reporters with UTR-9 inserted at different positions in the Adh coding region. The distance from TSS to AAUAAA is indicated below each schematic. (B) Northern blots of total RNA and poly(A)⁺ RNA of S2 cells transfected with the reporters shown in panel A; the probes used were those described for Fig. 2. (Left panel) The top band (with double-arrowed line) is the readthrough mRNA processed at the intergenic Adh poly(A) signal. Truncated transcripts processed at early poly(A) sites are indicated (P1, P2, and P3); ΔP2 and ΔP3 indicate mRNA derived by the deletion derivative lacking the initial region of Adh. (C) Agarose gel showing the DNA fragments produced by the adaptor RT-PCR assay of total RNA extracted from cells transfected with the indicated reporters. (D) Location of the poly(A) sites in the P1, P2, and P3 transcripts shown in panel C (based on sequencing of several clones of the P1, P2, and P3 RT-PCR fragments).