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J. Bacteriol. doi:10.1128/JB.01600-06
Copyright (c) 2006, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Porins are required for uptake of phosphates, by Mycobacterium smegmatis

Department of Microbiology, University of Alabama at Birmingham, 609 Bevill Biomedical Research Building, 845 19th Street South, Birmingham, AL 35294, U.S.A., Lehrstuhl für Mikrobiologie, Friedrich-Alexander-Universität Erlangen-Nürnberg, Staudtstr. 5, D-91058 Erlangen, Germany

^* To whom correspondence should be addressed. Email: mnieder{at}uab.edu.

	Abstract

Phosphorus is an essential nutrient, but it is unknown how phosphates are taken up across the mycobacterial cell wall. The phosphatase activity in whole cells of Mycobacterium smegmatis was significantly lower than in lysed cells indicating that access to the substrate was restricted. Loss of the outer membrane (OM) porin MspA also reduced the phosphatase activity of whole cells compared to lysed cells. A similar result was obtained for M. smegmatis which overexpressed the endogenous alkaline phosphatase indicating that PhoA is not a surface protein contrary to a previous report. Uptake of phosphate by a mutant lacking the porins MspA and MspC was two-fold slower compared to wild-type M. smegmatis. Strikingly, loss of these porins resulted in a severe growth defect of M. smegmatis on low phosphate plates. It is concluded that the OM of M. smegmatis represents a permeability barrier for phosphates and that the Msp porins are the only OM channels for diffusion of phosphate in M. smegmatis. However, phosphate diffusion through Msp pores is rather inefficient as shown by the 10-fold lower permeability of M. smegmatis for phosphate compared to glucose. This is likely due to the negative charges in the constriction zone of Msp porins. The phosphatase activity of whole cells of M. bovis BCG was significantly less than that of lysed cells indicating a similar uptake pathway for phosphates in slowly growing mycobacteria. However, porins which could mediate diffusion of phosphates across the OM of M. bovis BCG and M. tuberculosis are unknown.

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