THE NONHISTONE CHROMATIN PROTEIN HMG1 PROTECTS LINKER DNA ON THE SIDEOPPOSITE TO THAT PROTECTED BY LINKER HISTONES

Linker histones and HMG1/2 constitute the two major proteins that bind to linker DNA in chromatin. While the location of linker histones on the nucleosome has attracted considerable research effort, only a few studies have addressed the location of HMG1 in the particles. In this study, we use a procedure based on micrococcal nuclease digestion of r econstituted nucleosomal particles to which HMG1 has been bound, follo wed by analysis of the protected DNA by restriction nuclease digestion , to locate the HMG1 binding site. Nucleosomal particles were reconsti tuted on a 235-base pair DNA fragment, which is known to be a strong n ucleosome positioning sequence. The results unequivocally show that HM G1 protects linker DNA on one side of the core particle. Importantly, and possibly of physiological relevance, the linker DNA site protected by HMG1 was located on the side opposite to that already shown to be protected by linker histone binding.