The nucleotide sequences of the env genes of seven bovine leukemia viruses and the encoded peptide sequence were compared, with the objective of (i) determining the genetic distance separating bovine leukemia virus isolates from different geographical regions, (ii) identifying particular amino acids that contribute to the sequential and conformational epitopes, and (iii) relating such epitopes to their projected position in a threedimensional model of the structure of the gp5l surface glycoprotein. Two bovine leukemia virus subgroups were clearly identified, a Japanese-American subgroup represented by strains XBLV-1, VdM, and FLK-BLV and a European subgroup by strains T15-2, LB285, and LBS9. It was possible to identify amino acids that were important in determining three of the epitopes (F, G, and H) recognized by neutralizing monoclonal and polyclonal antibodies. On the model, these epitopes were adjacent and located on the exposed region of the molecule. Amino acid sequences contributing to a fourth cryptic epitope were identified; as predicted by the model, they lay on the opposite side to the neutralizable epitopes in a region involved in glycoprotein subunit association. The fact that this region is not normally exposed on the virion surface provides further evidence for the validity of the model.

C 25H29NO 10" HC1. C4H 10 O, monoclinic, P2~, a = 17.389 (8), b = 5.557 (3), c = 16.405 (8) A and fl = 91.41 (8) °, Z = 2. The structure was solved by direct methods and refined to an R of 0.07 for 2611 unique diffractometer data. The conformation and the interactions present in the complex structure allow us to propose a model for intercalation with DNA.

Sequence analysis of the endoglucanase EGCCA of Clostridium cellulolyticum indicates the existence of two domains: a catalytic domain extending from residue 1 to residue 376 and a reiterated domain running from residue 390 to 450. A small deletion in the C terminal end of the catalytic domain inactivated the protein. From the analysis of the sequences of 26 endoglucanases belonging to family A, we focused on seven amino acids which were totally conserved in all the catalytic domains compared. The roles of two of these, Arg-79 and His-122, were studied and defined on the basis of the mutants obtained by introducing various substitutions. Our findings suggest that Arg-79 is involved in the structural organization of the protein; the His-122 residue seems to be more essential for catalysis. The role of His-123, which is conserved only in subfamily A4, was also investigated.Clostridium cellulolyticum is a mesophilic anaerobic bacterium which is able to degrade crystalline cellulose (13). Studies of the various components of the cellulolytic complex of this bacterium are in progress, and previous studies have dealt with the cloning (8) and sequencing (9) of the celCCA gene, which encodes the endoglucanase EGCCA. Numerous genes of cellulases have now been sequenced, and the corresponding enzymes have been divided into nine different families on the basis of sequence comparisons and hydrophobic cluster analysis (14,17,18). According to this classification, EGCCA is a member of family A. At least 26 genes of cellulases belonging to family A have been sequenced to date, but none of their three-dimensional (3-D) structures have been determined. At present, the only 3-D structure described in the literature is that of cellobiohydrolase II from Trichoderma reesei (25), which belongs to family B. A second structure description should be published soon, as Clostridium thermocellum endoglucanase CELD (family E) has been crystallized (19) and its 3-D structure has been determined (20). Since the structure of the cellulases of family A is unknown, information about the structurefunction relationships must be deduced from the primary sequences. Using hydrophobic cluster analysis, Henrissat et al. (17) found that five segments were conserved, including several amino acids which were particularly well conserved. In the present study, we explored the roles of three of these amino acids: arginine 79 and histidine 122, which are conserved throughout the family, and histidine 123, which is conserved only in subfamily A4. MATERIALS AND METHODSStrains and media. Escherichia coli TG1 (30)

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2628THE OD STRUCTURE OF Na2SnS3 ions is 2.895 A, and the slight elongation compared with Na(1) is probably due to strains caused by the strong Sn-S bonds. The only deviation of the Na(2) and Na(3) arrangement from the L,, symmetry Plll(3-)mmm is that Na(3) has been shifted by a distance of 0.058(3) A. This effect, which is evident from the results of the refinement, has its chemical explanation in the fact that Na(3) has the two neighbouring Na atoms in cis position, while they are in trans position for Na(2) (Fig. 12) The structure of ellipticin (5,11-dimethyl-6H-pyrido[4,3-b]carbazole), a new antitumoral molecule, was solved by direct methods. The crystal belongs to the monoclinic system, space group P21/c (Z= 4) with cell parameters a=5,105, b= 15.588, e= 16.161 ~ and/~=97.03 °. The main feature of crystal packing appears to be different from that observed for some other dye molecules.