The gene encoding pilin of Bacteroides nodosus 340 has been isolated and the nucleotide sequence determined. The gene is present as a single copy within the B. nodosus genome and a protein of Mr 16683 can be predicted from the proposed coding region. A comparison of the predicted amino acid sequence with pilin from other strains of B. nodosus indicated that the protein of strain 340 (serogroup D) has a high degree of similarity with pilin of strain 265 (serogroup H). The degree of similarity between pilins from these strains and from other B. nodosus serogroups is no greater than that between B. nodosus pilins and the homologous proteins of several different bacterial species. These findings suggest that serogroups D and H may form a subset of B. nodosus serogroups.

Summary. Kinetic parameters on dihydroorotate dehydrogenase (DHO-DHase) from the rodent malarial parasite, Plasmodium berghei, have been determined. This enzyme, the fourth in de novo pyrimidine biosynthesis, is particulate and is absent in the mature mammalian red cell. The K,,, of the substrate, dihydroorotate, was determined to be 23 MM and the Ki values for a number of substrate analogues have been determined. The most potent inhibitor was dihydroazaorotate (Kl, 3 MM). The product orotate was also a good inhibitor (Ki, 5 MM) as were methylorotate (Ki, 10 /"M), 5-azaorotate (Ki, 20 MM) and other pyrimidine analogues. The activity of the enzyme was also affected by a number of respiratory chain inhibitors.As the P. berghei infection is accompanied by reticulocytosis, a comparative study of DHO-DHase in mouse reticulocytes was also carried out. The general properties of the enzyme from these sources were similar to those of the parasite enzyme. However, significant differences in the response of the two enzymes to various inhibitors were observed and could provide a rational basis for the development of chemotherapeutic agents active against the parasite.

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Sequences of pilin genes from four strains of serogroup B of the ovine pathogen Bacteroides nodosus have been determined. These sequences permit comparisons of amino acid sequence between pilins from different subtypes (B1, B2, B3, B4) of the B serogroup and assessment of intraserogroup variation. Pili of B. nodosus strains 234 (B1) and 183 (B2) were produced by Pseudomonas aeruginosa harboring a plasmid-borne B. nodosus pilin gene, and these pili were used in sheep vaccination trials. Pili from strain 183 (B2) were found to be a senior antigen to pili from strains of other B subtypes, providing protection against footrot infection caused by strains of the other B subtypes. Pili of this strain are therefore the most suitable candidate for inclusion in a pilus-based vaccine. Pili of strain 234 from subtype B1, the reference strain of the B serogroup, provided poor protection against infection with other subtypes.