Structure and expression of a phosphate deficiency-inducible ribonuclease gene in Pholiota nameko

Thirty-one cDNAs corresponding to pdi genes [inorganic phosphate (Pi) deficiency-inducible genes] were previously isolated through the differential screening of a cDNA library constructed from the mycelium of Pholiota nameko. Among the cDNAs, pdi370 was analyzed here. The deduced amino acid sequence showed high similarity to fungal ribonucleases (RNases) and contained two signature sequences conserved in T2 family RNases: CAS1 and CAS2. Genomic DNA harboring the pdi370 gene was isolated from a genomic library of P. nameko. Sequence analysis showed that the pdi370 gene is interrupted by 16 introns and that the promoter region contains two cis-acting sequences found in Pi deficiency-induced genes from Saccharomyces cerevisiae, together with several known functional elements, such as a TATA box. RNase activity in the mycelium and culture filtrate increased 5.6-fold and 5.2-fold, respectively, under Pi-deficient conditions. Staining for RNase activity showed that at least four RNases are induced and secreted under the conditions. The N-terminal sequence of one of them agreed with that of the pdi370 gene product.