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The crystal structure of aspergillopepsin I (AP) from Aspergillus phoenicis has been determined at 2.18 A Ê resolution and re®ned to R and R free factors of 21.5 and 26.0%, respectively. AP has the typical two -barrel domain structure of aspartic proteinases. The structures of the two independent molecules are partly different, exemplifying the¯exible nature of the aspartic proteinase structure. Notably, the`¯ap' in one molecule is closer, with a largest separation of 4.0 A Ê , to the active site than in the other molecule. AP is most structurally homologous to penicillopepsin (PP) and then to endothiapepsin (EP), which share sequence identities of 68 and 56%, respectively. However, AP is similar to EP but differs from PP in the combined S1H ±S2 subsite that is delineated by a¯exible 2-loop in the C-terminal domain. The S1H and S2 subsites are well de®ned and small in AP, while there is no de®nite border between S1H and S2 and the open space for the S2 subsite is larger in PP. Comparison of the structures indicates that the two amino-acid residues equivalent to Leu295 and Leu297 of AP are the major determining factors in shaping the S1H ±S2 subsite in the fungal aspartic proteinases.

Dihydrofolate reductase (DHFR) from bacteriophage T4 is a homodimer consisting of 193-residue subunits. It has been crystallized in the presence of the cofactor (NADPH) and an inhibitor (aminopterin) at 296 K using sodium chloride as precipitant. The crystals are tetragonal, belonging to the space group P4 1 22 (or P4 3 22), with unit-cell parameters a = b = 61.14, c = 123.23 A Ê under cryogenic conditions. The asymmetric unit contains a single subunit, with a corresponding V m of 2.65 A Ê 3 Da À1 and a solvent content of 53.6%. Native data have been collected from a crystal to 1.9 A Ê resolution using synchrotron X-rays.

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