The molecular link between tyrosol binding to tri6 transcriptional regulator and downregulation of trichothecene biosynthesis.

Tri cluster is responsible for the biosynthesis of trichothecenes in Trichoderma spp. Tri6 gene present within the cluster encodes for a transcriptional regulator and is vital for the expression of all other tri genes of the cluster. Tri6 encodes for a 218-amino-acid residues long protein which contains three zinc finger motifs. Tri6 is able to regulate and bind within the GTGA/TCAC promoter region of tri genes. Here we highlight the binding of tri6 with the regulatory DNA element present at the upstream of tri3 gene and effect of two quorum-sensing molecules tyrosol and farnesol on its binding. Analysis showed that tyrosol binds at sequence GTGA/TCAC specific for tri6 binding and thus do not allow tri6 to bind on promoter region. Interactions of tyrosol with zinc finger motif of tri6 protein also resulted in structural changes making tri6 unsuitable for binding with the regulatory DNA element of tri3 gene promoter resulting in downregulation of the gene. Structural changes also resulted in loss of zinc from zinc finger 2 motif. In contrast to the tyrosol, tri6-DNA complex could easily accommodate farnesol molecule without showing any interference with the functional conformation of the tri6-DNA complex. Therefore, tri gene expression seems not to be negatively regulated by the farnesol.