## 3 Results
### Sequence Analysis of SpFPPS Protein  
The molecular weight of SpFPPS was calculated to be 40.33 kDa using the ProtParam 3.0. The theoretical pI value is 5.11, encoding a protein of 350 amino acids. The sequence includes 55 negatively and 42 positively charged amino acids. The protein instability index is 43.55, suggesting that SpFPPS is an unstable protein. ProtScale indicated that the protein is a hydrophilic protein (Figure S3A). The TMHMM Server (v.2.0) predicted that the protein has no transmembrane structure.  
The secondary structure of SpFPPS predicted by PredictProtein included \(\alpha\)-helixes (63.43%), \(\beta\)-sheets (2.86%), extended strands (7.43%) and random coils (26.29%) (Figure S3B). The main structure is an \(\alpha\)-helix, which forms a cavity, and FARM and SARM are located on the two \(\alpha\)-helixes opposite. SWISS-MODEL was used to construct a 3D model [25, 26]. Based on the prediction of the X-ray and homologous proteins, the SpFPPS model has a sequence similarity of 51.17% with the template protein (PDB:1 FPS1); it was visualized using Pymol (Figure S4).  
To further explore the molecular characteristics of the expressed _SpFPPS_ gene expression products, we used DNAMAN to compare SpFPPS with FPPSs obtained from different species in the NCBI databank (Figure 1A). The results indicated that the structure contains five conserved regions; regions II and VI are aspartate acid-rich motifs designated FARM and SARM, respectively. The two conserved motifs are located on opposite walls of the central cavity facing each other (Figure S4). The FARM sequence is DDX (2-4) D (where X is any amino acid), which is defined further by upstream residues, while the SARM (DDXXD) is not. The FARM and the upstream five amino acids form a chain-length determination domain (CLD) [27]. According to the CLD sequence, FPPS is divided into three types. Type I FPPS, derived from eukaryote FPPS, has a FARM motif (DDXXD) and two aromatic amino acid residues at the fourth and fifth positions upstream of FARM. Type II FPPS (eubacterial) has a four-amino acid insertion in FARM (DDXXXXD) and a single aromatic amino acid at the fifth site upstream of FARM. Type III FPPS, the FARM motif, has a DDXD with only one aromatic amino acid that is located at the fifth position [27]. In SpFPPS, a type I FPPS,phenylalanine (F) and Y residues are located at the fourth and fifth positions upstream of FARM, respectively; the X amino acids are M (Figure 1A).  
Figure 1: Sequence analysis of FPPS proteins. (**A**) Sequence comparison of FPPSs from seven species. The red box represents FARM and SARM, and the yellow box represents CLD; (**B**) Construction of a phylogenetic tree based on FPPS amino acid sequences of different species using the neighbor-joining method. Red dots marked the SpFFPS in this study.  
The SpFPPS protein sequence was compared with the FPPS protein sequences of 19 different species identified in the NCBI database to construct a phylogenetic tree. The results showed that the FPPS of _S. pararoseus_ had the highest sequence similarity to that of _Rhodotorula toruloides_ (79.02%) and belonged to the same group as the FPPSs of fungi (Figure 1B).