THE QUTA ACTIVATOR AND QUTR REPRESSOR PROTEINS OF ASPERGILLUS-NIDULANS INTERACT TO REGULATE TRANSCRIPTION OF THE QUINATE UTILIZATION PATHWAY GENES

Genetic evidence suggests that the activity of the native QUTA transcr iption activator protein is negated by the action of the QUTR transcri ption repressor protein. When Aspergillus nidulans was transformed wit h plasmids containing the wild-type qutA gene, transformants that cons titutively expressed the quinate pathway enzymes were isolated. The co nstitutive phenotype of these transformants was associated with an inc reased copy number of the transforming qutA gene and elevated qutA mRN A levels. Conversely, when A, nidulans was transformed with plasmids c ontaining the qutR gene under the control of the constitutive pgk prom oter, transformants with a super-repressed phenotype (unable to utiliz e quinate as a carbon source) were isolated, The super-repressed pheno type of these transformants was associated with an increased copy numb er of the transforming qutR gene and elevated qutR mRNA levels. These copy-number-dependent phenotypes argue that the levels of the QUTA and QUTR proteins were elevated in the high-copy-number transformants. Wh en diploid strains were formed by combining haploid strains that conta ined high copy numbers of either the qutA gene (constitutive phenotype ) or the qutR gene (super-repressing; non-inducible phenotype), the re sulting diploid phenotype was one of quinate-inducible production of t he quinate pathway enzymes, in a manner similar to wild-type. The simp lest interpretation of these observations is that the QUTR repressor p rotein mediates its repressing activity through a direct interaction w ith the QUTA activator protein. Other possible interpretations are dis cussed in the text. Experiments in which truncated versions of the QUT A protein were produced in the presence of a wild-type QUTA protein in dicate that the QUTR repressor protein recognizes and binds to the C-t erminal half of the QUTA activator protein.