The Spore-Specific Alanine Racemase of Bacillus anthracis and Its Role in Suppressing Germination during Spore Development

Abstract

Spores of Bacillus anthracis are enclosed by an exosporium composed of a basal layer and an external hair-like nap. The nap is apparently formed by a single glycoprotein, while the basal layer contains many different structural proteins and several enzymes. One of the enzymes is Alr, an alanine racemase capable of converting the spore germinant l-alanine to the germination inhibitor d-alanine. Unlike other characterized exosporium proteins, Alr is nonuniformly distributed in the exosporium and might have a second spore location. In this study, we demonstrated that expression of the alr gene, which encodes Alr, is restricted to sporulating cells and that the bulk of alr transcription and Alr synthesis occurs during the late stages of sporulation. We also mapped two alr promoters that are differentially active during sporulation and might be involved in the atypical localization of Alr. Finally, we constructed a Deltaalr mutant of B. anthracis that lacks Alr and examined the properties of the spores produced by this strain. Mature Deltaalr spores germinate more efficiently in the presence of l-alanine, presumably because of their inability to convert exogenous l-alanine to d-alanine, but they respond normally to other germinants. Surprisingly, the production of mature spores by the Deltaalr mutant is defective because approximately one-half of the nascent spores germinate and lose their resistance properties before they are released from the mother cell. This phenotype suggests that an important function of Alr is to produce D-alanine during the late stages of sporulation to suppress premature germination of the developing spore.