Point mutations and DNA rearrangements 5' to the inducible qa-2 gene of Neurospora allow activator protein-independent transcription

Expression of the qa-2 gene of Neurospora crassa normally requires a functional activator protein encoded by qa-1F. Twelve transcriptional mutants of the qa-2 gene have been isolated in qa-1F-strains, and these allow partial expression of qa-2 (1-45% of induced wild type) in the absence of functional activator protein. All 12 mutants have been characterized by genomic (Southern) blot hybridization and the DNAs of 5 have been cloned and sequenced. Eight mutations consist of large DNA rearrangements within a 500-base-pair region 5′to the qa-2 gene. One large rearrangement mutation, located 378 base pairs before the normal site of transcription initiation, causes exceptional levels of qa-2 transcription (45% of induced wild type) from near the normal initiation site. Two of the other four mutations cloned involve tandem duplications (68 and 84 base pairs) of the same upstream region (centered at nucleotide - 145), and two involve ``point'' mutations (at nucleotides -200 and -95) that closely flank the duplicated region. With one possible exception, none of the mutations appears to involve changes directly associated with RNA polymerase II binding and hence they differ from analogous mutations in comparable prokaryotic systems. The overall results suggest that at least some of the large DNA rearrangement mutations may be acting as upstream activator elements, possibly by juxtaposing enhancer-like sequences, whereas the duplications and point mutations may define a region of qa-2 regulation, for instance at the level of RNA polymerase II access.