Rearrangement mutations on the 5' side of the qa-2 gene of Neurospora implicate two regions of qa-1F...

Transcriptional activation of the Neurospora crassa qa genes normally requires the positive regulatory gene, qa-1F+, whose function is controlled by the inducer quinic acid and by the product of the negative regulatory gene, qa-1S+. The properties of qa-1F+ activator have been examined in transcriptional mutations of the qa-2 structural gene, in which activator-independent transcription of qa-2 (qa-2ai mutants) occurs in strains having a qa-1F- gene. Seven qa-2ai mutants with DNA rearrangements in different 5' regions of qa-2 were analyzed in qa-1F+ strains. In five with rearrangements at position -190 or further upstream, expression of the qa-2 gene was inducible, and induction was accompanied by a change in the initiation site for transcription from position -45, characteristic of constitutive initiation in qa-2ai mutants to position +1, characteristic of the induced wild type. In two mutants with breakpoints at positions -86 and -53, qa-2 transcription initiated from upstream sequences within the rearrangements but not at the +1 site, and qa-2 expression was noninducible. The results indicate that (i) sequences between positions -190 and -86 are required for positive control of initiation at position +1, and (ii) negative control does not require sequences downstream of position -86. Additional evidence suggests that the product of the qa-1F+ gene in the noninduced state may also interact with distal upstream sequences positioned midway between divergently transcribed qa genes.