Systematic variation in DNA length yields highly ordered repressor-operator cocrystals.

Crystals have been grown that contain the operator-binding domain of the lambda repressor and the lambda operator site OL1. Crystallization conditions were tested with a set of DNA fragments, ranging in length from 17 to 23 base pairs. The best crystals were grown with a 20-base pair DNA fragment. These crystals have space-group symmetry P2I, with unit cell dimensions a = 37.1 A, b = 68.8 A, c = 56.8 A, and a beta angle of 91.5 degrees. They diffracted to at least 2.5 A resolution. High resolution data from these crystals should allow the direct determination of how a repressor recognizes its operator site.