1. Mg2+-limited Bacillus subtilis var. niger, growing in a chemostat in a simple salts medium, contained considerably more potassium and phosphorus than Mg2+-limited Aerobacter aerogenes growing in a similar medium at corresponding dilution rates. 2. Growth of the bacillus in a K+-limited environment did not lower the cellular potassium and phosphorus contents, the molar proportions of cell-bound magnesium, potassium, RNA (as nucleotide) and phosphorus being approximately constant at 1:13:5:13 (compared with 1:4:5:8 in Mg2+-limited or K+-limited A. aerogenes). 3. Growth of B. subtilis in a phosphate-limited environment caused the cellular phosphorus content to be lowered to a value similar to that of Mg2+-limited A. aerogenes, but the potassium content was not correspondingly lowered; the molar potassium:magnesium ratio varied from 14 to 17 with changes in dilution rate from 0·4 to 0·1hr.−1. 4. Whereas over 70% of the cell-bound phosphorus of Mg2+-limited or K+-limited A. aerogenes was contained in the nucleic acids, these polymers accounted for less than 50% of the phosphorus present in similarly limited B. subtilis; much phosphorus was present in the walls of the bacilli, bound in a teichoic acid-type compound composed of glycerol phosphate and glucose (but no alanine). 5. Phosphate-limited B. subtilis cell walls (from organisms grown at a dilution rate of 0·2hr.−1) contained little phosphorus and no detectable amounts of teichoic acid, but 40% of the cell-wall dry weight could be accounted for by a teichuronic acid-type compound; this contained a glucuronic acid and galactosamine, neither of which could be detected in the walls of Mg2+-limited B. subtilis grown at a corresponding rate. 6. It is suggested that the high concentration of potassium in growing B. subtilis (compared with A. aerogenes) results from the presence of large amounts of anionic polymer (teichoic acid or teichuronic acid) in the bacillus cell walls.

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