Date Published: October 15, 2018
Publisher: Springer Berlin Heidelberg
Author(s): Wen Tang, Changsheng Ouyang, Lanlan Liu, Haoran Li, Chuanhui Zeng, Jie Wang, Lijun Fu, Qinqin Wu, Bin Zeng, Bin He.
Fatty acid desaturases play a key role in producing polyunsaturated fatty acids by converting single bonds to double bonds. In the present study, a total of 13, 12, 8 and 8 candidate fatty acid desaturases genes were identified in the Aspergillus oryzae, Aspergillus flavus, Aspergillus fumigatus and Aspergillus nidulans genomes through database searches, which were classified into five different subfamilies based on phylogenetic analysis. Furthermore, a comprehensive analysis was performed to characterize conserved motifs and gene structures, which could provide an intuitive comprehension to learn the relationship between structure and functions of the fatty acid desaturases genes in different Aspergillus species. In addition, the expression pattern of 13 fatty acid desaturases genes of A. oryzae was tested in different growth stages and under salt stress treatment. The results revealed that the fatty acid desaturases genes in A. oryzae were highly expressed in adaptive phase growth and up-regulated under salt stress treatment. This study provided a better understanding of the evolution and functions of the fatty acid desaturases gene family in the four Aspergillus species, and would be useful for seeking methods to improve the production of unsaturated fatty acids and enhance efforts for the genetic improvement of strains to adapt to the complex surrounding environment.
Unsaturated fatty acids, which contain one or more double bonds, are the major structural components of cell membranes. Therefore, they play significant roles in maintaining cell structure and the membrane fluidity, which are involved in development, energy metabolism and stress response (Pereira et al. 2003). Unsaturated fatty acids are synthesized by individual fatty acid desaturases via introducing double bonds into the hydrocarbon chains of fatty acids (Chi et al. 2011; Shanklin and Cahoon 1998). Fatty acid desaturases are found in almost all organisms, including plants, animals, bacteria and fungi. According to localization and cofactor requirements, fatty acid desaturases have been broadly classified into two evolutionary groups: soluble and membrane-bound desaturases. The soluble fatty acid desaturases, such as the plant Acyl-carrier-protein (ACP) desaturase family, use acyl carrier protein thioesters as substrates, and use ferredoxin oxidoreductase and ferredoxin as electron donors. The membrane-bound fatty acid desaturases, which include Δ5-, Δ6-, Δ9-, Δ12- and Δ15-desaturase in the mammals, fungi, insects, higher plants and cyanobacteria, use fatty acids esterified to complex lipid as the substrate, and use cytochrome (cyt) b5 oxidoreductase and cyt b5 as electron donors. In addition, most of fatty acid desaturases share three highly conserved histidine boxes: ʻHXXXXHʼ, ʻHXXHHʼ and ʻQXXHHʼ included in the fatty acid desaturases domain. The fatty acid desaturases domain was the essential domain of the fatty acid desaturases gene family. However, some researches revealed that the cytochrome b5 domain play a key role in the synthesis of unsaturated fatty acids as an electron donor to activate desaturase (Pereira et al. 2003). Zhang et al. reported that the cytochrome b5 is required for biosynthesis of polyunsaturated fatty acids in Caenorhabditis elegans (Zhang et al. 2005). Although fatty acid desaturases orthologs of different organisms share some obvious structure characteristics, the structural and functional features were distinctive among plants, animals and fungi. Most studies of fatty acid desaturases gene family focused on plants and animals. For example, Liu et al. characterized 19 genes encoding fatty acid desaturases and analyzed their expression profiles in Gossypium raimondii under low temperature (Liu et al. 2015). Xue et al. cloned and characterized fatty acid desaturases gene family from Salvia hispanica and Perilla frutescens (Xue et al. 2018).
Studies have indicated that fatty acid desaturases is expressed in plants, animals and fungi, and plays an important role in the synthesis of polyunsaturated fatty acid (Garba et al. 2016; Murphy and Piffanelli 1998). Fatty acid desaturases genes in fungi are responsible for multiple biological processes, from development and industrial production to adaption to the surrounding environment (Watanabe et al. 2010). Therefore, the study of the fatty acid desaturases genes is becoming a hot spot in the current biological research. So far, researchers have used DNA library, cDNA library and RT-PCR to clone fatty acid desaturases genes from plants, animals, bacteria, fungi and algae. For example, 29 and 21 fatty acid desaturases gene members were respectively identified from the fatty acid desaturases gene families of the soybean and Arabidopsis thaliana (Chi et al. 2011). Besides, there are some reports on the cloning and expression of fatty acid desaturases in microbes. For example, the sole fatty acid desaturases gene in Bacillus subtilis, named des, encoding Δ5-desaturase, was cloned by Ma and Liu (Ma and Liu 2010). The expression of three fatty acid desaturases genes in the Cytosolic cyanobacteria, named desA, desB and desD, was up-regulated under low temperature (Los et al. 1997). Aspergillus species has been intensively used for the production of traditional fermented foods and secondary metabolite, such as fumagillin. Therefore, the synthesis of unsaturated fatty acid appears to be essential and vital for the Aspergillus species to adapt to some complex environments and regulate the growth as well as development. However, systematic investigations of the fatty acid desaturases gene family at the whole-genome level was absent in Aspergillus species. Therefore, a comprehensive survey of fatty acid desaturases gene family in Aspergillus species was undertaken.