Abstract
It is well known that the Na+ cycle and the cell wall are essential for alkaline adaptation of Na+-dependent alkaliphilic Bacillus species. In Bacillus pseudofirmus 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 B. pseudofirmus OF4. However, other than SlpA, the functions of SLH domain-containing proteins are not well known. Therefore, a deletion mutant of the csaB gene, required for the retention of SLH domain-containing proteins on the cell wall, was constructed to investigate its physiological properties. The csaB mutant strain of B. pseudofirmus 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 slpA mutant strain. Ultra-thin section transmission electron microscopy showed that a csaB mutant strain lacked an S-layer part, and its peptidoglycan (PG) layer was disturbed. The slpA 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 B. pseudofirmus OF4 play an important role in alkaline adaptation via peptidoglycan synthesis.
Highlights
The typical Na+-dependent alkaliphilic Bacillus species require Na+ for growth and motility (Krulwich et al, 2001, 2011; Ito et al, 2011)
Since protoplasts of Na+-dependent alkaliphilic Bacillus halodurans C-125 are unstable in an alkaline environment, the cell wall is considered to be important for alkaline adaptation (Aono et al, 1992)
We report microscopic observations of the cell morphology, cell wall components, and cell growth at different pH values of B. pseudofirmus OF4 and its surface layer (S-layer) protein mutant strains to elucidate the effects of S-layer homology (SLH) domain-containing proteins on alkaline adaptation and cell morphology
Summary
The typical Na+-dependent alkaliphilic Bacillus species require Na+ for growth and motility (Krulwich et al, 2001, 2011; Ito et al, 2011). Since protoplasts of Na+-dependent alkaliphilic Bacillus halodurans C-125 are unstable in an alkaline environment, the cell wall is considered to be important for alkaline adaptation (Aono et al, 1992). The cell wall has two major kinds of acidic polymers called teichuronic acid (TUA) and teichuronopeptide (TUP) (Aono, 1985). TUA and TUP are considered to contribute to cell adaptation to an alkaline environment. It was reported that the amounts of TUA and TUP are enhanced in cells grown at an alkaline pH, as compared to those grown at neutral pH (Aono, 1985), and the mutants deficient in TUA and TUP have lost alkaliphilicity (Aono and Ohtani, 1990; Aono et al, 1995, 1999; Ito and Aono, 2002)
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