Characterization of cotJ, a sigma E-controlled operon affecting the polypeptide composition of the coat of Bacillus subtilis spores

Abstract

The outermost protective structure found in endospores of Bacillus subtilis is a thick protein shell known as the coat, which makes a key contribution to the resistance properties of the mature spore and also plays a role in its interaction with compounds able to trigger germination. The coat is organized as a lamellar inner layer and an electron-dense outer layer and has a complex polypeptide composition. Here we report the cloning and characterization of an operon, cotJ, located at about 62 degrees on the B. subtilis genetic map, whose inactivation results in the production of spores with an altered pattern of coat polypeptides. The cotJ operon was identified by screening a random library of lacZ transcriptional fusions for a conditional (inducer-dependent) Lac+ phenotype in cells of a strain in which the structural gene (spoIIGB) for the early-acting, mother-cell-specific transcriptional factor sigma E was placed under the control of the IPTG (isopropyl-beta-D-thiogalactopyranoside)-inducible Pspac promoter. Sequence analysis of cloned DNA from the cotJ region complemented by genetic experiments revealed a tricistronic operon preceded by a strong sigma E-like promoter. Expression of an SP beta-borne cotJ-lacZ fusion commences at around h 2 of sporulation, as does expression of other sigma E-dependent genes, and shows an absolute requirement for sigma E. Studies with double-reporter strains bearing a cotJ-gusA fusion and lacZ fusions to other cot genes confirmed that expression of cotJ is initiated during sporulation prior to activation of genes known to encode coat structural proteins (with the sole exception of cotE). An in vitro-constructed insertion-deletion mutation in cotJ resulted in the formation of spores with no detectable morphological or resistance deficiency. However, examination of the profile of electrophoretically separated spore coat proteins from the null mutant revealed a pattern that was essentially identical to that of a wild-type strain in the range of 12 to 65 kDa, except for polypeptides of 17 and 24 kDa, the putative products of the second (cotJB) and third (cotJC) cistrons of the operon, that were missing or reduced in amount in the coat of the mutant. Polypeptides of the same apparent sizes are detected in spores of a cotE null mutant, on which basis we infer that the products of the cotJ operon are required for the normal formation of the inner layers of the coat or are themselves structural components of the coat. Because the onset of cotJ transcription is temporally coincident with the appearance of active sigma E, we speculate that the cotJ-encoded products may be involved in an early state of coat assembly.