The Surface Layer Homology Domain-Containing Proteins of Alkaliphilic Bacillus pseudofirmus OF4 Play an Important Role in Alkaline Adaptation via Peptidoglycan Synthesis

It is well known that the Na cycle and the cell wall are essential for alkaline adaptation of Na -dependent alkaliphilic species. In OF4, surface layer protein A (SlpA), the most abundant protein in the surface layer (S-layer) of the cell wall, is involved in alkaline adaptation, especially under low Na concentrations. The presence of a large number of genes that encode S-layer homology (SLH) domain-containing proteins has been suggested from the genome sequence of OF4. However, other than SlpA, the functions of SLH domain-containing proteins are not well known. Therefore, a deletion mutant of the gene, required for the retention of SLH domain-containing proteins on the cell wall, was constructed to investigate its physiological properties. The mutant strain of OF4 had a chained morphology and alkaline sensitivity even under a 230 mM Na concentration at which there is no growth difference between the parental strain and the mutant strain. Ultra-thin section transmission electron microscopy showed that a mutant strain lacked an S-layer part, and its peptidoglycan (PG) layer was disturbed. The mutant strain also lacked an S-layer part, although its PG layer was not disturbed. These results suggested that the surface layer homology domain-containing proteins of OF4 play an important role in alkaline adaptation via peptidoglycan synthesis.