3.1 Sequence screening
In the sequence mining, the prokaryote-derived SuSy templatesAn SuSy and Mr SuSy were used for sequence collection and the sequences without conservative residues G302, G303, H438, R580, L581, K585, and E675,[22] and residues Q648, N654, and E683 that contribute to UDPG binding were removed (the residue number refers to At SuSy1 in the multiple sequence alignment).[11, 24] As a result, only a few sequences from lower eukaryote sources like green algae remained together with a large number of the putative plant SuSys. Mc SuSy from M. conductrix and Cb SuSy1 and Cb SuSy2 fromC. braunii were selected and synthesized after codon optimization for heterologous expression in E. coli . Enzyme activities of the crude extracts containing Mc SuSy and Cb SuSy2 were around 15.5 and 5.5 mU/mg total protein, respectively. However, it is difficult to detect the SuSy activity of Cb SuSy1. As well, the obvious band corresponding to Mc SuSy (Fig. S1) was found in the soluble fraction prepared from the induced cells, indicating the better soluble expression of recombinant Mc SuSy than the other two enzymes. Therefore, Mc SuSy was chosen for further study of enzymatic properties.