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Spores of Bacillus anthracis are enclosed by an exosporium composed of a basal layer and an external hair-like nap. The nap is formed by a collagen-like glycoprotein called BclA, while the basal layer contains many different proteins, one of which is a spore-specific alanine racemase (Alr). In this study, we employed fluorescence microscopy and a fluorescently labelled anti-Alr monoclonal antibody (mAb) to examine the distribution of Alr within the exosporium. Binding of the mAb occurred over approximately three-quarters of the exosporium but not in a cap-like region at one end of the spore, indicating the absence or inaccessibility of Alr in this region. We also determined that the cap-like region, or cap, corresponds to the first part of the exosporium assembled within the mother cell during sporulation and the only part of the exosporium assembled in a DeltaexsY mutant strain of B. anthracis. Our results provide the first direct evidence that exosporium assembly is a non-uniform process and suggest that exosporium formation is discontinuous. Finally, we demonstrated that during spore germination and outgrowth, the outgrowing cell always escapes from its exosporium shell by popping through the cap, suggesting that the cap is designed to facilitate the emergence of the outgrowing cell.

Listeria monocytogenes is an intracellular bacterial pathogen that elicits a strong cellular immune response following infection and therefore has potential use as a vaccine vector. However, while infections by L. monocytogenes are fairly rare and can readily be controlled by a number of antibiotics, the organism can nevertheless cause meningitis and death, particularly in immunocompromised or pregnant patients. We therefore have endeavored to isolate a highly attenuated strain of this organism for use as a vaccine vector. D-Alanine is required for the synthesis of the mucopeptide component of the cell walls of virtually all bacteria and is found almost exclusively in the microbial world. We have found in L. monocytogenes two genes that control the synthesis of this compound, an alanine racemase gene (dal) and a D-amino acid aminotransferase gene (dat). By inactivating both genes, we produced an organism that could be grown in the laboratory when supplemented with D-alanine but was unable to grow outside the laboratory, particularly in the cytoplasm of eukaryotic host cells, the natural habitat of this organism during infection. In mice, the double-mutant strain was completely attenuated. Nevertheless, it showed the ability, particularly under conditions of transient suppression of the mutant phenotype, to induce cytotoxic T-lymphocyte responses and to generate protective immunity against lethal challenge by wild-type L. monocytogenes equivalent to that induced by the wild-type organism.

Limited proteolysis studies on alanine racemase suggested that the enzyme subunit is composed of two domains (Galakatos, N. G., and Walsh, C. T. (1987) Biochemistry 26, 8475-8480). We have constructed a mutant gene that tandemly encodes the two polypeptides of the Bacillus stearothermophilus enzyme subunit cleaved at the position corresponding to the predicted hinge region. The mutant gene product purified was shown to be composed of two sets of the two polypeptide fragments and was immunologically identical to the wild-type enzyme. The mutant enzyme, i.e. the fragmentary alanine racemase, was active in both directions of the racemization of alanine. The maximum velocity (Vmax) was about half that of the wild-type enzyme, and the Km value was about double. Absorption and circular dichroism spectra of the fragmentary enzyme were similar to those of the wild-type enzyme. An attempt was made to separately express in Escherichia coli a single polypeptide corresponding to each domain, but no protein reactive with the antibody against the wild-type alanine racemase was produced. Therefore, it is suggested that the two polypeptide fragments can fold into an active structure only when they are co-translated and that they correspond to structural folding units in the parental polypeptide chain.