to a superfamily of 8-fold /]/a-barrels with similar amino acid residues at their active sites. In the three families that these enzymes represent, the nucleophile is a glutamate, which is located close to the carboxy-terminus of/]-strand seven. In addition all three enzymes have the sequence asparagine-glutamate close to the carboxy-terminus of /]-strand four. This glutamate has been identified as the acid/base in the family F xylanases and is essential for catalysis in /]-galactosidase. We suggest that the equivalent residue in the barley glucanases is the acid/base. Analysis of the sequences of family 1 /]-glucosidases and family 5 cellulases shows that these enzymes also belong to this superfamily which we call the 4/7 superfamily.

The architecture of P. fluorescens subsp. cellulosa has been determined crystallographically to be a commonly occurring enzyme fold, the eight-fold alpha/beta-barrel. Xylopentaose binds across the carboxy-terminal end of the alpha/beta-barrel in an active-site cleft which contains the two catalytic glutamates.

We have solved the structure of the Bacillus subtilis pectate lyase (BsPel) in complex with calcium. The structure consists of a parallel beta-helix domain and a loop region. The alpha L-bounded beta-strand seen in BsPel is a new element of protein structure and its frequent occurrence suggests it is an important characteristic of the parallel beta-helix. A pronounced cleft is formed between the loops and the parallel beta-helix domain and we propose that this is the active site cleft. Calcium, essential for the activity of the enzyme, binds at the bottom of this cleft and an arginine residue close to the calcium, which is conserved across all pectin and pectate lyases, may be involved in catalysis.

851possible, but provides a very simple and accurate method if it is. The systematic ge6metry also has the advantage of providing many independent structure factors, unlike the more accurate critical voltage method, which provides only a relationship between structure factors. The intersecting-Kikuchi-line method can also provide useful approximate values of structure factors, and is by far the easiest to apply since it relies on distance measurements taken from film, rather than intensity measurements. AbstractThe anisotropic displacements of selected rigid groups in bovine ribonuclease A have been refined from X-ray diffraction data by the application of the * Present address:

The atomic displacements of many of the atoms in a macromolecular structure can be modelled in terms of group motions described in the harmonic approximation by T, L and S tensors. Relevant groups may be planar side groups of protein chains, units of secondary structure such as o~-helices or whole protein domains. For the TLS parameters to be interpreted, they must be related to the axes of inertia of the rigid groups and, in the case of the T and S tensors, must be calculated with respect to the centre of reaction of the rigid group. A program (TLSANL)is described that analyses these 21 TLS rigid-body displacement parameters and their relation with the principal axes of the rigid body, from the output of the segmented anisotropic refinement of a macromolecular structure, as produced by a program such as RESTRAIN