Mechanisms controlling the two phosphate uptake systems in Neurospora crassa

The development of the high-affinity and low-affinity phosphate uptake systems of Neurospora crassa has been followed during germination and early growth. The ratio between the activities of the two systems became constant by the time exponential growth began, although the value of this ratio depended on the external phosphate concentration. The regulatory mechanisms controlling the systems were investigated by following the changes that resulted when exponentially growing germlings adapted to one phosphate concentration were shifted to a different concentration. The high-affinity system was derepressed under conditions of phosphate starvation, and inhibited irreversibly by feedback inhibition under conditions of over-supply. The low-affinity system was also derepressed and subject to feedback inhibition under comparable conditions, but, in contrast, inhibition of this system was reversible. A detailed description is given of the interplay between the systems during adaptation to changes in phosphate supply. Changes that occurred in the internal phosphate pool support the hypothesis that this metabolite is responsible for controlling the activities of the systems, either by triggering derepression of new uptake system synthesis or by inhibiting the existing system by feedback.