The Neurospora crassa metallothionein gene. Regulation of expression and chromosomal location.

The promoter region of the Neurospora crassa metallothionein gene contains no sequences which are similar to the mammalian or the yeast metal responsive elements (Münger, K., Germann, U. A., and Lerch, K. (1985) EMBO J. 4, 2665-2668). We therefore studied the regulation of expression of the N. crassa metallothionein gene in response to different metal ions (Cu2+, Cd2+, Zn2+, Co2+, and Ni2+) by Northern analysis. Only copper led to the induction of metallothionein mRNA. In N. crassa cultures inoculated and grown in copper-supplemented media, metallothionein mRNA appeared during the late logarithmic growth period (about 30 h after inoculation) and was detectable for a time period of more than 30 h. In response to copper shock, however, rapidly increasing amounts of metallothionein mRNA were detected within minutes after copper administration at any time in vegetatively growing mycelia of N. crassa. Maximum levels were detected about 1 h after addition of copper to the medium. The half-life time of the mRNA was estimated as 2.5 h. The amounts of copper metallothionein reach a maximum level at 3 h after induction and thereafter remain constant. The rapid induction by copper ions of metallothionein mRNA and metallothionein together with the remarkable stability of the native protein intracellularly suggest that this protein serves an important homeostatic role in the copper metabolism in this fungus. The structural gene of N. crassa metallothionein has been located on chromosome VI using restriction fragment-length polymorphisms as genetic markers.