The Janus transcription factor HapX controls fungal adaptation to both iron starvation and iron excess.
Bottom Line: We further demonstrate that a HapX homodimer and the CCAAT-binding complex (CBC) cooperatively bind an evolutionary conserved DNA motif in a target promoter.The latter reveals the mode of discrimination between general CBC and specific HapX/CBC target genes.Collectively, our study uncovers a novel regulatory mechanism mediating both iron resistance and adaptation to iron starvation by the same transcription factor complex with activating and repressing functions depending on ambient iron availability.
Affiliation: Division of Molecular Biology, Biocenter, Innsbruck Medical University, Innsbruck, Austria.Show MeSH
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Mentions: To identify putative, evolutionary conserved, regulatory motifs in the cccA promoter, the 1-kb 5′-upstream regions of cccA homologs from 28 fungal species including A. fumigatus, A. nidulans and F. oxysporum were subject to MEME analysis (Bailey & Elkan, 1994). The identified sites and their positions in the promoters of the different species are shown in Supplementary Fig S4. The highest scoring sequence (e-value of 3.4 × 10−115), present in all 28 species, was a bipartite motif separated by a spacer region with low conservation (Fig6A). Consistent with the HapX-independent regulation, the highest scoring motif was not found in the promoter of the S. cerevisiae cccA homolog (data not shown). The 5′-conserved submotif matches the CBC consensus DNA-binding motif (CCAAT box), CCAAT(C/T)(A/G) (Huber et al, 2012). This is in perfect agreement with the previous finding that HapX acts via physical interaction with the CBC (Hortschansky et al, 2007). Interestingly, binding of the CCAAT box by CBC would cover the entire spacer region as identified in the CBC/DNA binary complex crystal structure (Huber et al, 2012), which indicates that the 3′-submotif is the first accessible region for binding of another DNA-binding protein. The 3′-conserved non-palindromic submotif does not match any known transcription factor consensus binding sequence. This is intriguing, because bZIP proteins usually bind short palindromic or pseudo-palindromic target sequences. Furthermore, based on the amino acid signature sequence of its basic region NXXAQXXFR (Supplementary Fig S1), HapX belongs to the Pap1/Yap1 subfamily of bZIP transcription factors that are known to recognize TTACGTAA and TTAGTAA consensus motifs (Fujii et al, 2000).
Affiliation: Division of Molecular Biology, Biocenter, Innsbruck Medical University, Innsbruck, Austria.