BLAST analysis of BTA genes

BLAST analysis of BTA1 genes from C. reinhardtii and K. lactis show that databases contain sequences of BTA1 orthologs from more than 30 species of basidiomycetes, all from fungal whole-genome projects. None of them has been functionally characterized. We demonstrated that BTA1 gene of basidiomycete F. velutipes is inducible by Pi deficiency. Unfortunately, the regulatory mechanism of BTA1 induction in somatostatin receptor has not yet been studied. It is known that DGTS synthesis during Pi limitation in ascomycete fungi is under the control of the PHO regulon, mediated by the transcription factor referred to as Pho4p in S. cerevisiae and NUC-1 in N. crassa (Riekhof et al., 2014). While the PHO pathway is widespread among ascomycete fungi (Tomar and Sinha, 2014), no information is available about the function of this pathway in basidiomycetes. Based on similarity searches, we could not identify clear orthologs of Pho4 and Pho2 proteins in complete genomes of basidiomycetes but found putative orthologs of other components of the PHO pathway: cyclin Pho80, cyclin-dependent kinase Pho85 and cyclin-dependent kinase inhibitor Pho81. However, the role of these proteins in phosphate starvation response in basidiomycete fungi has not been elucidated. Additionally, there is evidence that in some ascomycete fungi, orthologs of the S. cerevisiae PHO pathway components can have another functions, for example, the cyclin-CDK complex SpPHO80-SpPHO85 in S. pombe (Henry et al., 2011). Thus, while the basidiomycete F. velutipes and the ascomycetes N. crassa and K. lactis demonstrate a similar phosphate starvation response by the replacement of phospholipid PC with non-phosphorus DGTS, it is unclear how the Pi starvation response is regulated in fungi. Another question concerns the mechanism of PC reduction during the phosphate starvation response. There are at least three main mechanisms of PC depletion: suppression of PC synthesis due to substrate exhaustion, downregulation of PC synthase expression by suppression of gene transcription or intensification of mRNA degradation as well as intensive hydrolysis of PC by phospholipases. We supposed that substitution of PC by DGTS is compensatory reaction directed on phosphate release for vitally important cell processes and regulated at the level of both DGTS and PC gene transcription. Therefore we expected the decrease of mRNA abundance of PC synthesis genes. However our data provides the first evidence that the PC biosynthesis genes CPT1 and CHO2 increase their expression under phosphate-limiting conditions. These results indicate that regulation by mRNA abundance does not contribute to PC depletion during Pi-starvation in basidial fungi. What is the mechanism of elevating of mRNA abundance of PC synthesis genes – activation of synthesis or increase of stability – are the questions for future studies. The present work broadens the scope of our knowledge on the adaptive mechanism that induce DGTS synthesis in response to Pi-deficiency.