Promoters (green) grab onto nuclear pores (red) only when activated (bottom).

LAEMMLI/ELSEVIER

Aquick visit to the nuclear pore is not an oddity but occurs for most if not all regulated genes, say Manfred Schmid, Ulrich Laemmli, and colleagues (University of Geneva, Switzerland). At least in yeast, the visit may be needed to boost activation of transcription.

The Geneva group had earlier found factors linking transcription activation to the nuclear pore. They now use a nuclease fused to a nuclear pore protein, which clips any DNA that comes to the nuclear pore for a visit.

The regions preferentially cleaved by the nuclease were just upstream of ∼40% of genes on chromosome VI. Constitutively expressed genes use different promoter elements and may not need the activation conferred by the visit. But back-of-the-envelope calculations suggest that there are enough pores for all activated genes to get in a quick visit, even if they do so for every round of transcription.

Laemmli suggests that “the pore is an active participant in forming a complex between enhancer and promoter.” Constrained enhancers could no longer influence more distant promoters.

In higher eukaryotes, Laemmli thinks that so-called transcription factories may act as promoter attachment and assembly sites, and thus as the intranuclear equivalent of nuclear pores. In yeast, the Geneva group is mutating a newly isolated gene that should confirm whether the visit to the pore is an essential event.

Reference:

Schmid, M., et al.
2006
.
Mol. Cell.
21
:
379
–391.