(A) Scheme of the glycosylation reaction based on which the high-throughput screening method was developed. (B) The representative activity data of glycosylation of fluorescent 4-MU illustrating ~100 random members from the GTF-D saturation mutagenesis library screening. The wild-type enzyme and mutant M4 were indicated. Strain BL21(DE3) harboring plasmid pET28a was used as control. Activities were calculated as the fluorescence differences of the variants between 0 (before the reaction) and 5 h (after the reaction).