GLOBAL REGULATION OF SULFUR ASSIMILATION IN NEUROSPORA

A complex regulatory circuit controls expression of many permeases and enzymes involved in sulfur assimilation in the filamentous fungus Neu rospora crassa. CYS3, the global positive-acting sulfur regulatory pro tein, turns on the expression of structural genes that encode sulfur e nzymes when N. crassa cells are limited for sulfur. Expression of the cys-3 gene itself is highly regulated by negative-acting sulfur-contro ller scon genes and by autogenous regulation. The CYS3 protein is loca lized within the nucleus and contains a bZip DNA-binding motif and reg ions rich in alanine and in proline that appear to function in trans a ctivation. Amino acid substitutions for basic or neutral amino acids i n the bZip domain of CYS3 lead to significant changes in its DNA-bindi ng activity.