Crystallization of a 67 kDa fragment of Escherichia coli DNA topoisomerase I

Escherichia coli DNA topoisomerase I is a well-studied type I DNA topoisomerase that catalyzes the breakage and rejoining of one DNA strand to allow passage of the other strand. We have cloned and over-expressed a 67 kDa amino-terminal fragment of the protein, and shown that it retains the ability of the intact enzyme to cleave single-stranded DNA. High-quality crystals of the purified 67 kDa fragment have been obtained. The crystals belong to space group P2₁2₁2₁, with cell dimensions a=64.0 Å, b=79.9 Å and c=142.3 Å. They diffract to at least 2.8 Å at low temperature and, when cooled to cryogenic temperatures, to at least 1.9 Å in a synchrotron source. A complete native data set and two derivative data sets have been collected. A multiple isomorphous replacement map to 3 Å resolution shows clear secondary structural elements. Final structure determination is in progress.