A bipartite yeast SSRP1 analog comprised of Pob3 and Nhp6 proteins modulates transcription

The FACT complex of vertebrate cells, comprising the Cdc68 (Spt16) and SSRP 1 proteins, facilitates transcription elongation on a nucleosomal template and modulates the elongation-inhibitory effects of the DSIF complex in vitr o. Genetic findings show that the related yeast (Saccharomyces cerevisiae) complex, termed CP, also mediates transcription. The CP components Cdc68 an d Pob3 closely resemble the FACT components, except that the C terminal hig h-mobility group (HMG) hox domain of SSRP1 is not found in the yeast homolo g Pob3, We show here that Nhp6a and Nhp6b, small HMG box proteins with over lapping functions in yeast, associate with the CP complex and mediate CP-re lated genetic effects on transcription. Absence of the Nhp6 proteins causes severe impairment in combination with mutations impairing the Swi-Snf chro matin-remodeling complex and the DSIF (Spt4 plus Spt5) elongation regulator , and sensitizes tells to 6-azauracil, characteristic of elongation effects , An artificial SSRP1-like protein, created by fusing the Pob3 and Nhp6a pr oteins, provides both Pob3 and Nhp6a functions for transcription, and compe tition experiments indicate that these functions are exerted in association with Cdc68. This particular Pob3-Nhp6a fusion protein was Limited for cert ain Nhp6 activities, indicating that its Nhp6a function is compromised. The se findings suggest that in yeast cells the Cdc68 partners may be both Pob3 and Nhp6, functioning as a bipartite analog of the vertebrate SSRP1 protei n.