Sin1p/Spt2p is a yeast chromatin protein that, when mutated or deleted, alters the transcription of a family of genes presumably by modulating local chromatin structure. In this study, we investigated the ability of different domains of this protein to bind four-way junction DNA (4WJDNA) since 4WJDNA can serve as a model for bent double helical DNA and for the crossed structure formed at the exit and entry of DNA to the nucleosomes. Sequence alignment of Sin1p/Spt2p homologues from 11 different yeast species showed conservation of several domains. We found that three domains of Sin1p/ Spt2p fused to glutathione S-transferase can each bind independently in a structure-specific manner to 4WJDNA as measured in a gel mobility shift assay. A feature common to these domains is a cluster of positively charged amino acids. Modification of this cluster resulted in either abolishment of binding or a change in the binding properties. One of the domains tested clearly bound superhelical DNA, although it failed to induce bending in a circularization assay. Poly-L-lysine, which may be viewed as a cluster of positively charged amino acids, bound 4WJDNA as well. Phenotypic analysis showed that disruption of any of these domains resulted in suppression of a his4-912␦ allele, indicating that each domain has functional significance. We propose that Sin1p/Spt2p is likely to modulate local chromatin structure by binding two strands of double-stranded DNA at their crossover point.Sin1p/Spt2p is a yeast chromatin non-histone protein. While the precise function of the protein is still unknown, it is known to function as a negative transcriptional regulator of a number of genes including SUC2 (1), INO1 (2), and SSA3 (3). Activity of the HO promoter, as measured from an HO promoter driving a lacZ gene (4), is also regulated by SIN1/SPT2. swi1, swi2, and swi3 mutants are unable to transcribe RNA from the HO promoter. However, in sin1/swi double mutants, transcription is restored. Mutants in SPT2 were first identified as suppressors of ty and ␦ insertions in the 5Ј non-coding region of the HIS4 gene (5). As the negative regulation of these genes is overcome by the SW1/SNF chromatin remodeling complex, it was suggested that a function of SIN1p/SPT2p is to somehow maintain chromatin compaction at specific locations in the chromatin.
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