Efficient transcription termination downstream
of poly(A) sites has been shown to correlate with the strength
of an upstream polyadenylation signal and the presence
of a polymerase pause site. To further investigate the
mechanism linking termination with 3′-end processing,
we analyzed the cis-acting elements that contribute
to these events in the Saccharomyces cerevisiae FBP1
gene. FBP1 has a complex polyadenylation signal,
and at least three efficiency elements must be present
for efficient processing. However, not all combinations
of these elements are equally effective. This gene also
shows a novel organization of sequence elements. A strong
positioning element is located upstream, rather than downstream,
of the efficiency elements, and functions to select the
cleavage site in vitro and in vivo. Transcription run-on
analysis indicated that termination occurs within 61 nt
past the poly(A) site. Deletion of two UAUAUA-type efficiency
elements greatly reduces polyadenylation in vivo and in
vitro, but transcription termination is still efficient,
implying that FBP1 termination signals may be
distinct from those for polyadenylation. Alternatively,
assembly of a partial, but nonfunctional, polyadenylation
complex on the nascent transcript may be sufficient to
cause termination.