Shape of ocr, the gene 0.3 protein of bacteriophage T7: Modeling based on light scattering experiments

Ocr, the first protein expressed by bacteriophage T7, inhibits type I DNA restriction enzymes by preventing them from binding to DNA. This inhibition allows the phage to successfully infect the host. The shape of ocr is modeled on the basis of static and dynamic light scattering measurements. The static light scattering data confirm previous observations that ocr exists in solution as a dimer. The diffusion constant determined by dynamic light scattering indicates a nonspherical shape of the ocr dimer. Hydrodynamic models of ellipsoids are presented, and it is argued that ocr is best described by a prolate ellipsoid with dimensions of 10.4 run by 2.6 mn. The size and shape predicted by this model are consistent with ocr acting as a mimic of the DNA structure bound by type I restriction enzymes.