Regulation of sulfate transport in neurospora by transinhibition and by inositol depletion

Neurospora possesses two distinct sulfate transport systems, a low-affinity form (Permease I) which is the only type found in conidia, and a second species (Permease II) which predominates during the mycelial stage. Although methionine represses the synthesis of both of these permeases, inorganic sulfate only partially represses the mycelial form and does not affect the synthesis of Permease I. Both transport systems are also regulated by transinhibition. The transinhibition which occurs in mycelia is not due to an intracellular pool of inorganic sulfate, but is instead exerted by an early intermediate of the sulfate assimilatory pathway.The development of functional sulfate transport activity depends upon genetic and metabolic events which affect the cell membrane. The synthesis of sulfate permease activity in the inos mutant requires an exogenous supply of inositol. The effect of the cot mutant, which is thought to interfere with membrane synthesis, also prevents the development of sulfate permease at the restrictive temperature. The maintenance of pre-existing functional sulfate permease activity apparently also requires a continuous renewal of membrane components since withdrawal of inositol from inos mutants results in a rapid inactivation of transport activity.