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Role of anionic phospholipids in the adaptation of Bacillus subtilis to high salinity

Claudia S. López^1,2,

Alejandro F. Alice^1,

¹ Facultad de Ciencias Exactas y Naturales de la Universidad de Buenos Aires, Departamento de Química Biológica, Ciudad Universitaria Pabellón II 1428, Buenos Aires, Argentina
² Instituto de Biología Celular y Neurociencias ‘Dr E. De Robertis’, Facultad de Medicina Universidad de Buenos Aires (UBA), Paraguay 2155 (1121), Buenos Aires, Argentina
³ Instituto de Investigaciones Bioquímicas de La Plata (INIBIOLP) (UNLP/CONICET), Facultad de Ciencias Médicas, Calle 60 y 120 (1900), La Plata, Argentina

Correspondence
Carmen Sánchez-Rivas
sanchez{at}qb.fcen.uba.ar

The importance of the content of anionic phospholipids [cardiolipin (CL) and phosphatidylglycerol (PG)] in the osmotic adaptation and in the membrane structure of Bacillus subtilis cultures was investigated. Insertion mutations in the three putative cardiolipin synthase genes (ywiE, ywnE and ywjE) were obtained. Only the ywnE mutation resulted in a complete deficiency in cardiolipin and thus corresponds to a true clsA gene. The osmotolerance of a clsA mutant was impaired: although at NaCl concentrations lower than 1·2 M the growth curves were similar to those of its wild-type control, at 1·5 M NaCl (LBN medium) the lag period increased and the maximal optical density reached was lower. The membrane of the clsA mutant strain showed an increased PG content, at both exponential and stationary phase, but no trace of CL in either LB or LBN medium. As well as the deficiency in CL synthesis, the clsA mutant showed other differences in lipid and fatty acids content compared to the wild-type, suggesting a cross-regulation in membrane lipid pathways, crucial for the maintenance of membrane functionality and integrity. The biophysical characteristics of membranes and large unilamellar vesicles from the wild-type and clsA mutant strains were studied by Laurdan's steady-state fluorescence spectroscopy. At physiological temperature, the clsA mutant showed a decreased lateral lipid packing in the protein-free vesicles and isolated membranes compared with the wild-type strain. Interestingly, the lateral lipid packing of the membranes of both the wild-type and clsA mutant strains increased when they were grown in LBN. In a conditional IPTG-controlled pgsA mutant, unable to synthesize PG and CL in the absence of IPTG, the osmoresistance of the cultures correlated with their content of anionic phospholipids. The transcriptional activity of the clsA and pgsA genes was similar and increased twofold upon entry to stationary phase or under osmotic upshift. Overall, these results support the involvement of the anionic phospholipids in the growth of B. subtilis in media containing elevated NaCl concentrations.

In memory of Dr Emilio A. Rivas.

Present address: Dept of Molecular Microbiology and Immunology, Oregon Health and Science University, Portland, OR, USA.

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