Upstream binding factor association induces large-scale chromatin decondensation.

Area of Science:

• Molecular Biology
• Chromatin Biology
• Gene Regulation

Background:

• The precise function of upstream binding factor (UBF), a key component of the RNA polymerase I preinitiation complex, remains largely undetermined.
• Recent findings indicate UBF's widespread distribution across ribosomal gene repeats, suggesting roles beyond promoter binding, potentially involving chromatin remodeling.

Purpose of the Study:

• To directly investigate the impact of UBF on chromatin structure using an in vivo assay.
• To elucidate the mechanisms by which UBF influences chromatin decondensation and transcription machinery recruitment.

Main Methods:

• An in vivo assay was employed, targeting UBF via a lac repressor fusion protein to a heterochromatic, amplified chromosome region containing lac operator repeats.
• Chromatin decondensation, recruitment of specific complexes (SL1, RNA polymerase I), and histone modifications were analyzed.

Main Results:

• UBF association with the targeted locus induced significant large-scale chromatin decondensation.
• This decondensation occurred independently of common remodeling complexes like SWI/SNF and histone acetyltransferases, and was not linked to histone H3 lysine 9 acetylation.
• UBF successfully recruited the RNA polymerase I-specific complex SL1 and RNA polymerase I subunits.

Conclusions:

• UBF's dynamic association with ribosomal DNA clusters is proposed to recruit the RNA polymerase I transcription machinery.
• This interaction maintains ribosomal DNA loci in a transcriptionally competent state.
• The study establishes an in vivo model for analyzing ribosomal DNA transactivation and transcription machinery assembly outside the nucleolus.