The enzyme RuvC Resolvase was co-crystallized with two different dna substrates. Each substrate is a four-way dna junction, better known as a Holliday Junction composed of opposite arms of six and eight base pairs. The two substrates differ in their dna base sequences and have been characterized biochemically by Benson et al..(JBC Vol 268,5195-5201). Substrate #1 is known to be resolved by RuvC in the vicinity of the 12 base pair core of the Holliday Junction. Substrate#2, which is deficient in THY in the sequence of the 12 base pair core, has been shown to be recognized by the RuvC enzyme but not cleaved. Four derivative data sets of varying quality were collected at CHESS Feb 1996. Two data sets were collected on iodine derivatives of chemically modified dna for Holliday Junction Substrate Type #1. The data for both collections is >98% complete at 3.3A with internal r-factors of 10.7% and 12.1% respectively. All of the Iodine sites could not be identified properly and hence neither Patterson has been solved to date. Two data sets were collected on Hg derivates of phospho-thioate modified dna of the second substrate type. The data for these derivatives is complete at 3A, however the internal r-factors are greater than 15%. At present we are pursuing the molecular placement strategy for determining the structure of the RuvC-dna complexes. We have generated a map which still requires some refinement of the protein and considerable rebuilding of the dna substrate. We anticipate revisiting the data we obtained at CHESS to confirm our results by molecular placement.