Polyadenylation and transcription termination in gene constructs containing multiple tandem polyadenylation signals

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Abstract
The processes of pre-mRNA 3′-end cleavage and polyadenylation have been closely linked to transcription termination by RNA polymerase II. We have studied the relationship between polyadenylation and transcription termination in gene constructs containing tandem poly(A) signals, at least one of which is the inefficient polyomavirus late poly(A) site. When identical tandem viral signals were separated by fewer than 400 bp, they competed for polyadenylation. The upstream site was always chosen preferentially, but relative site choice was influenced by the distance between the signals. All of these constructs showed the same low level of transcription termination as wild type polyomavirus, which contains a single late poly(A) site. When tandem poly(A) signals were not identical, a stronger downstream signal could outcompete a weaker upstream signal for polyadenylation without altering the efficiency of transcription termination characteristic for use of the upstream signal. Thus, if a weak polyoma virus late poly(A) signal (associated with inefficient transcription termination) preceded a strong rabbit β-globin signal (associated with efficient transcription termination), termination remained inefficient, but the distal signal was most often chosen for polyadenylation. These results are consistent with independent regulation of polyadenylation and transcription termination in this system and are discussed in light of current models for the dependence of transcription termination on a functional poly(A) site.