A role for upstream binding factor in organizing ribosomal gene chromatin

Area of Science:

• Molecular Biology
• Human Genetics
• Chromosomal Structure

Background:

• Human ribosomal genes reside in nucleolar organizer regions (NORs) on acrocentric chromosomes.
• Active NORs manifest as secondary constrictions during metaphase, characterized by achromatic banding and positive silver staining.
• Upstream binding factor (UBF), an architectural RNA polymerase I transcription factor, extensively binds ribosomal gene repeats.

Purpose of the Study:

• To investigate the role of UBF in underpinning NOR structure.
• To examine the formation of novel chromosomal structures induced by ectopic UBF-binding sequences.

Main Methods:

• Analysis of human cell lines engineered with large arrays of heterologous UBF-binding sequences integrated at ectopic chromosomal sites.
• Assessment of UBF recruitment to these ectopic sites.
• Morphological and silver-staining characterization of resulting chromosomal features during metaphase.

Main Results:

• Ectopic UBF-binding sequence arrays efficiently recruited UBF, even outside the nucleolus.
• These arrays formed novel, silver-stainable secondary constrictions during metaphase, termed pseudo-NORs.
• Pseudo-NORs exhibited morphological similarities to endogenous NORs.

Conclusions:

• UBF plays a fundamental role in establishing and maintaining NOR architecture.
• The formation of pseudo-NORs provides direct evidence for UBF's structural role in organizing ribosomal gene loci.
• This study elucidates a key mechanism governing the higher-order structure of human ribosomal gene regions.