Spermidine Synthase and Saccharopine Reductase Have Co-Expression Patterns Both in Basidiomycetes with Fusion Form and Ascomycetes with Separate Form

Gene fusion is a process through which two or more distinct genes are fused into a single chimeric gene. Unlike most harmful fusion genes in cancer cells, in this study, we first found that spermidine synthetase- (SPDS, catalyst of spermidine biosynthesis) and saccharopine reductase- (SR, catalyst of the penultimate step of lysine biosynthesis) encoding genes form a natural chimeric gene, FfSpdsSr, in Flammulina filiformis. Through the cloning of full-length ORFs in different strains and the analysis of alternative splicing in developmental stages, FfSpdsSr has only one copy and unique transcript encoding chimeric SPDS-SR in F. filiformis. By an orthologous gene search of SpdsSr in more than 80 fungi, we found that the chimeric SpdsSr exists in basidiomycetes, while the two separate Spds and Sr independently exist in ascomycetes, chytridiomycetes, and oomycetes. Further, the transcript level of FfSpdsSr was investigated in different developmental stages and under some common environmental factors and stresses by RT-qPCR. The results showed that FfSpdsSr mainly up-regulated in the elongation stage and pileus development of F. filiformis, as well as under blue light, high temperature, H(2)O(2), and MeJA treatments. Moreover, a total of 15 sets of RNA-Seq data, including 218 samples of Neurospora crassa, were downloaded from the GEO database and used to analyze the expression correlation of NcSpds and NcSr. The results showed that the separate NcSpds and NcSr shared highly similar co-expression patterns in the samples with different strains and different nutritional and environmental condition treatments. The chimeric SpdsSr in basidiomycetes and the co-expression pattern of the Spds and Sr in N. crassa indicate the special link of spermidine and lysine in fungi, which may play an important role in the growth and development of fruiting body and in response to the multiple environmental factors and abiotic stresses.