Molecular Cloning and Regulatory Analysis of the Arylsulfatase Structural Gene of Neurospora crassa

Abstract

The ars-1+ gene of Neurospora crassa encodes the enzyme arylsulfatase. ars-1+ is in a group of highly regulated sulfur-related structural genes that are expressed under conditions of sulfur limitation and are under coordinate control of the cys-3+ and scon+ regulatory genes. The ars-1+ gene was cloned by chromosome walking from the qa gene cluster, using a lambda library. Cotransformation of an N. crassa ars-1 mutant with the isolated lambda clones and the benomyl resistance gene, followed by assay for arylsulfatase activity, was used to screen for the ars-1+ gene. Further confirmation that the cloned segment mapped to the ars-1+ locus was obtained by restriction-fragment-length polymorphism analysis. Northern (RNA) blot analysis showed that the ars-1+ gene was transcribed to give an mRNA of 2.3 kilobases. In wild-type cells, the ars-1+ transcript was abundant under sulfur-derepressing conditions but absent under repressing conditions. Time course analysis showed that the appearance of ars-1+ message in sulfur-derepressed cultures paralleled the appearance of arylsulfatase enzyme activity. In addition, transcription of ars-1+ was detected only under derepressing conditions in a nuclear transcription assay. In a cys-3 regulatory mutant that was unable to synthesize arylsulfatase (or other sulfur-controlled enzymes), there was no ars-1+ transcript under repressing or derepressing conditions. In a temperature-sensitive cys-3 mutant, the ars-1+ transcript was present only at the permissive growth temperature and under sulfur derepression. A negative regulatory mutant, sconc, displayed both constitutive expression of arylsulfatase enzyme activity and content of ars-1+ message.