DNA LOOPING BY SACCHAROMYCES-CEREVISIAE HIGH-MOBILITY GROUP PROTEINS NHP6A B - CONSEQUENCES FOR NUCLEOPROTEIN COMPLEX ASSEMBLY AND CHROMATIN CONDENSATION/

The formation of higher order protein . DNA structures often requires bending of DNA strands between specific sites, a process that can be f acilitated by the action of nonspecific DNA-binding proteins which ser ve as assembly factors. A model for this activity is the formation of the invertasome, an intermediate structure created in the Hin-mediated site-specific DNA inversion reaction, which is stimulated by the prok aryotic nucleoid-associated protein HU. Previously, we have shown that the mammalian HMG1/2 proteins substitute for HU in this system and di splay efficient DNA wrapping activity in vitro. In the present work, w e isolate the primary sources of assembly factor activity in Saccharom yces cerevisiae, as measured by the ability to stimulate invertasome f ormation, and show that these are the previously identified NHP6A/B pr oteins. NHP6A/B have comparable or greater activity in DNA binding, be nding, and supercoiling with respect to HU and HMG1 and appear to form more stable protein . DNA complexes. In addition, expression of NHPA in mutant Escherichia coli cells lacking HU and Fis restores normal mo rphological appearance to these cells, specifically in nucleoid conden sation and segregation. From these data we predict diverse architectur al roles for NHP6A/B in manipulating chromosome structure and promotin g the assembly of multicomponent protein . DNA complexes.