As the genetic blueprint of life, DNA must be deciphered or "read," even when densely packed into nucleosomes. The nucleosome is therefore a key target of genetic processes in a cell and a focus of scientific investigations into how normal and diseased cells work. Previous studies at Penn State and other research institutions led to the discovery of chromatin enzymes -- proteins that act to turn specific genes on or off by binding to the nucleosome. Since the three-dimensional structure of the nucleosome was determined 13 years ago, scientists have wondered how chromatin enzymes recognize and act on the nucleosome to regulate gene expression and other processes in a cell.
To tackle this problem, Ravindra D. Makde, a postdoctoral member of the research team led by Tan, grew molecular crystals of the protein RCC1 (regulator of chromosome condensation, a protein critical for proper separation of chromosomes during cell division) bound to the nucleosome, and used X-ray crystallography to determine the atomic structure of the complex. The structure provides atomic details of how an enzyme can recognize both DNA and components of the protein core of the nucleosome. Unexpectedly, the structure also showed how DNA can stretch as it wraps into a nucleosome.