Zinc Stimulation of Nitrogen Assimilation by Helminthosporium Cynodontis

SUMMARY

Zinc increased the uptake and retention of ammonia-nitrogen (NH₃-N) concomitant with increased growth of a strain of Helminthosporium cynodontis. Zinc also stimulated increased growth with nitrate or nitrite as the nitrogen source, although this organism exhibits preferential utilization of NH₃-N. While NH₃-N uptake in the presence of glucose was identical in pregrown zinc-replete and zinc-deficient mycelia, only zinc-replete mycelia retained significant quantities following efflux of the NH₃-N in ammonia-free buffer. In the absence of glucose, neither zinc-replete nor zinc-deficient mycelia retained an appreciable amount of the NH₃-N. Exogenous zinc addition to pregrown zinc-deficient mycelia had no effect on either the uptake or retention of NH₃-N. The uptake of NH₃-N by this organism is apparently a passive process, but its retention requires the presence of a zinc-dependent assimilative capacity. Glutamic dehydrogenase activity of zinc-replete mycelia was significantly higher than that from zinc-deficient mycelia and the specific activity increased when increasing concentrations of zinc were added to the growth medium. The data suggest a role for zinc in the incorporation of NH₃-N through glutamic dehydrogenase in this organism.