Colletotrichum trifolii TB3 kinase, a COT1 homolog, is light inducible and becomes localized in the nucleus during hyphal elongation

Colletotrichum trifolii is a fungal pathogen responsible for anthracnose disease of alfalfa. Previously, a serine/threonine protein kinase gene from this fungus (TB3), which is a functional homolog of the Neurospora crassa COT1 kinase, has been isolated in our laboratory and appears to be associated with hyphal elongation and branching. In this report we show that light treatment rapidly induces TB3 expression and hyphal branching frequency. Western analysis showed TB3 localization in both the cytoplasm and nucleus, but not in membranes. Moreover, indirect immunofluorescence indicated that TB3 levels were most abundant in the nucleus. To further evaluate the subcellular distribution of TB3, a TB3::GFP fusion construct was inserted into C. trifolii. Results indicated that the cellular location of TB3 changed during fungal growth and development. Consistent with previous observations, TB3 was localized in both the cytoplasm and the nucleus but was preferentially localized in the nucleus during extended hyphal growth. The amino terminus of TB3 contains two relatively long polyglutamine repeats. Yeast-based assays showed that these polyglutamine tracts can activate transcription. These results suggest that TB3 may be positioned in a signaling cascade regulating proper hyphal growth and development by functioning as a transcription factor.