Regulation of constitutively expressed and induced cutinase genes by different zinc finger transcription factors in Fusarium solani f. sp. pisi (Nectria haematococca)

Cutin monomers, generated by the low levels of constitutively expressed cutinase, induce high levels of cutinase that can help pathogenic fungi to penetrate into the host through the cuticle whose major structural polymer is cutin. We cloned three highly homologous cutinase genes, cut1, cut2, and cut3, from Fusarium solani f. pisi (Nectria haematococca). Amino acid sequence deduced from the nucleotide sequence of cut1 and cut2/3 matched with that of the peptides from cutinase 1 and cutinase 2, respectively, isolated from F. solani pisi grown on cutin as the sole carbon source. Induction of β-glucuronidase gene fused to the promoters of the cutinases integrated into F. solani pisi genome indicates that cut2 is constitutively expressed and induced under starvation, whereas cut1 is highly induced by cutin monomers. A palindrome binding protein (PBP) previously cloned binds only to palindrome I of cut1 promoter but not palindrome 1 of cut2/3 which contains two base substitutions. PBP is thought to interfere with the binding of CTF1α, the transcription factor involved in induction, to cut1 promoter and thus keep cut1 gene repressed until induced by cutin monomers. Because PBP cannot bind palindrome 1 of cut2, this gene is not repressed. CTF1α does not transactivate cut2 promoter. A new Cys₆Zn₂ motif-containing transcription factor, CTF1β, that binds palindrome 2 was cloned and sequenced. In yeast, CTF1β transactivates cut2 promoter but not cut1 promoter unless its palindrome 1 is mutated, unlike CTF1α which transactivates cut1. Thus, CTF1β is involved in the constitutive expression of cut2 that causes production of low levels of cutin monomers that strongly induce cut1 using CTF1α as the transcription factor.