Institute of High Energy Physics, Chinese Academy of Sciences^* Institute of Physics, Chinese Academy of Sciences, P. R. China^** Faculty of Advanced Life Sciences, Hokkaido University, Japan^***
○Deqiang Yao^* Yuanxin Gu^** Chaode Zheng^** Zhengjiong Lin^** Haifu Fan^** Nobuhisa Watanabe^***

Case studies for SAD phasing and fragment extension by OASIS-2004 with ~2.0Å data have been reported previously. The present study is to probe the low resolution limit that allows a successful direct-method SAD phasing. Sulfur-SAD data of the 69 kDa protein TT0570 were collected with in-house Cu-Ka radiation at 1.8Å resolution. Its truncated subsets at 2.1, 3.5 and 4.0Å resolution were used in the test. The sulfur substructure was solved at each resolution independently. The data has an expected Bijvoet ratio <|DF|>/<F> ~ 0.56%. In the 2.1 Å case, a single run of OASIS+DM+ARP/wARP led automatically to a model containing 1178 of the total 1206 residues all docked in sequence. In the 3.5Å case, two cycles iteration of OASIS+DM+RESOLVE (build only) resulted in a model with more than 700 residues, 58 among them were docked in sequence. The corresponding electron density map is traceable by an experienced worker. In the 4.0Å case, a single run of OASIS+DM+RESOLVE (build only) gave a somewhat degraded map, which failed to be improved by further iteration but still provides useful information.