Summary: The best labelling of (var. ) with P was achieved when the organisms were grown in broth containing 33 μg. orthophosphate/ml. and 150--200 μc./ml. P. Cocci labelled in this way were as virulent to mice as control cocci. The phosphorus content of cocci was proportional to the orthophosphate content of the medium over the range 5 μg.-300 μg. PO/ml. P was released from cocci when they were incubated with phosphate buffer and other media which did not support cell division. P was released from labelled dividing cocci grown in non-radioactive media. About 80% of the P was released during division; a maximum of about 55% was released from non-dividing cocci over a similar time period. Most of the P released from dividing cocci was present in the form of molecules non-diffusible in dialysis (80%); most of the P released into phosphate buffer was diffusible. Cocci deficient or rich in phosphorus were fractionated with trichloroacetic acid (TCA), ether, ethanol and perchloric acid. Cocci containing low amounts of P lacked P in the fraction soluble in cold TCA, about 80% of which was ortho-phosphate. Cocci deficient in phosphorus incorporated P from PO solutions more rapidly than enriched cocci. The distributions of P in cocci previously rich or deficient in P were similar, but labelling in deficient cocci was increased in the RNA-containing fraction; and that part of the fraction soluble in hot TCA which had a barium salt insoluble at pH 4. The phosphorus level in the cells had no effect on the amount of phosphorus released into phosphate buffer solution.


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