The Interactive Fly

Evolutionarily conserved developmental pathways

General gene activation machines: NURF and SWI/SNF

Chromosomes are made up of chromatin, a complex of DNA and proteins. Chromatin maintains genes in an inactive state, unaccessable to gene activating transcriptions factors. In recent years, an understanding has developed regarding molecular machines that are able to use the energy of the cell in order to remodel chromatin. In these evolutionarily conserved machines are responsible for gene activation. The prototypical chromatin remodeling molecular machine is SWI/SNF (pronounced switch-sniff) from yeast. At least five proteins constitute the complex. In the presence of ATP (a source of energy for breaking chemical bonds), SWI/SNF is able to remodel chromatin, modifying the access of transcription factors to DNA regulatory regions.

Two multiprotein complexes exist in Drosophila. NURF (Nucleosome Remodeling Factor) contains the protein ISWI that possesses a DNA-dependent ATPase domain, characteristic of SWI2 protein family members. Yeast SWI2 is a component of yeast SWI/SNF complex, the prototypical chromatin remodeling molecular machine. Brahma and SNR1 and Moira are three components of a second Drosophila chromatin remodeling molecular machine. Brahma has both a helicase domain of unknown function and a bromodomain, presumably functioning in protein-protein interaction. The homology of Brahma to Drosophila ISWI is high in the helicase domain, but ISWI, with a lower molecular weight than Brahma, lacks the C-terminal bromodomain. The Drosophila SNR1 protein is a homolog of the yeast SNF5, another subunit of the yeast SWI/SNF complex, and Moira is a homolog of yeast SWI3, a third component of the yeast SWI/SNF complex..

The ATPase activity of NURF, the first Drosophila chromatin remodeling machine to be characterized, is activated by chromatin, while the ATPase activity of the complex containing ISWI and Brahma is activated by naked DNA. Thus there appear to be two SWI/SNF related complexes in Drosophila, each serving a different function. At this point the functional differences between the two complexes are just beginning to be understood.

date revised: 13 Oct 96

Developmental Pathways conserved in Evolution

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