Regulation of Phosphate Accumulation in the Unicellular CyanobacteriumSynechococcus

Abstract

The P contents of acid-soluble pools, lipid, RNA and acid-insoluble polyphosphate were lowered in Synechococcus in proportion to the reduction in growth rate in phosphate-limited but not in NO3-limited continuous culture. P in these cell fractions was lost proportionately during progressive Pi starvation of batch cultures. Acid-insoluble polyphosphate was always present in all cultural conditions to about 10% of total cell P and did not turn over during balanced exponential growth. Extensive polyphosphate formation occurred transiently when Pi was given to cells which had been Pi limited. This material was broken down after 8 h even in the presence of excess external orthophosphate, and its P was transferred into other cell fractions, notably RNA. Pi uptake kinetics indicated an invariant apparent Km of about 0.5 .mu.M, but Vmax was 40-50 times greater in cells from Pi-limited cultures than in cells from NO3-limited or balanced batch cultures. Over 90% of the Pi taken up within the first 30 s at 15.degree. C was recovered as orthophosphate. The uptake process is highly specific, since neither Pi entry nor growth was affected by a 100-fold excess of arsenate. The activity of polyphosphate synthetase in cell extracts increased at least 20-fold during Pi starvation or in Pi-restricted growth, but polyphosphatase activity was little changed by different growth conditions. Derepression of the Pi transport and polyphosphate-synthesizing systems as well as alkaline phosphatase occurs in Pi shortage, but that the breakdown of polyphosphate in this organism is regulated by modulation of existing enzyme activity.