Spt16-Pob3 and the HMG protein Nhp6 combine to form the nucleosome-bindingfactor SPN

Yeast Spt16/Cdc68 and Pob3 form a heterodimer that acts in both DNA replica tion and transcription. This is supported by studies of new alleles of SPT1 6 described here. We show that Spt16-Pob3 enhances HO transcription through a mechanism that is affected by chromatin modification, since some of the defects caused by mutations can be suppressed by deleting the histone deace tylase Rpd3, While otherwise conserved among many eukaryotes, Pob3 lacks th e HMG1 DNA-binding motif found in similar proteins such as the SSRP1 subuni t of human FACT. SPT16 and POB3 display strong genetic interactions with NH P6A/B, which encodes an HMG1 motif, suggesting that these gene products fun ction coordinately in vivo, While Spt16-Pob3 and Nhp6 do not appear to form stable heterotrimers, Nhp6 binds to nucleosomes and these Nhp6-nucleosomes can recruit Spt16-Pob3 to form SPN-nucleosomes. These complexes have alter ed electrophoretic mobility and a distinct pattern of enhanced sensitivity to DNase I. These results suggest that Spt16-Pob3 and Nhp6 cooperate to fun ction as a novel nucleosome reorganizing factor.