A strain-variable bacteriocin in Bacillus anthracis and Bacillus cereus with repeated Cys-Xaa-Xaa motifs

Abstract

Bacteriocins are peptide antibiotics from ribosomally translated precursors, produced by bacteria often through extensive post-translational modification. Minimal sequence conservation, short gene lengths, and low complexity sequence can hinder bacteriocin identification, even during gene calling, so they are often discovered by proximity to accessory genes encoding maturation, immunity, and export functions. This work reports a new subfamily of putative thiazole-containing heterocyclic bacteriocins. It appears universal in all strains of Bacillus anthracis and B. cereus, but has gone unrecognized because it is always encoded far from its maturation protein operon. Patterns of insertions and deletions among twenty-four variants suggest a repeating functional unit of Cys-Xaa-Xaa.

Figure 1

Genomic context shows evidence for a "heterocycloanthracin" group of bacteriocin precursors. This shows comparable genomic regions in Bacillus anthracis Ames and Bacillus licheniformis ATCC 14580. Black signifies proposed protoxin, green for cyclodehydratase, blue for docking scaffold, yellow for dehydrogenase, and gray a Bacillus-specific unknown protein. The # character indicates a gene prediction we designate NT03BL1107.1, missed in public annotation.

Figure 2

Sequence alignment shows evidence for a "heterocycloanthracin" group of bacteriocin precursors. This shows an alignment of protoxin sequences. Insertions and deletions of the form Cys-Xaa-Xaa, and Cys-Gly-Gly in particular, account for many of the differences between non-identical sequence pairs. Shown are Bacillus anthracis protein BA_2677 (RF|YP_019318.1), twenty-two non-identical close homologs from other Bacillus strains, and the more distantly related protoxin from B. licheniformis. The Arg-Arg pair represents the boundary between the N-terminal (leader peptide) and C-terminal (Cys-rich) regions. At least one heterocycloanthracin is found in every completed genome studied from B. anthracis, B. cereus, B. thuringiensis, and B. weihenstephanensis. Strains of origin are B. anthracis Ames (sequence 1), B. cereus E33l (2), B. c. H3081.97 (3), B. thuringiensis sv. konkukian str. 97–27 (4), B. c. ATCC 14579 (5, 18), B. weihenstephanensis KBAB4 (6,15), B. c. NVH0597-99 (7), B. c. AH187 (8, 23), B. t. sv. israelensis ATCC 35646 (9, 20), B. c. G9842 (10, 22), B. c. G9241 (11, 21), B. c. ATCC 10987 (12, 17), B. c. B4264 (13, 19), B. c. subsp. cytotoxis NVH 391-98 (14), B. c. AH1134 (16), and B. licheniformis ATCC 14580 (24).