Methods for preparing improved enzyme variants

The present invention relates to a high throughput screening method for preparing a variant of catalytic polypeptide capable of catalyzing a chemical reaction. The method of selecting a bacterium comprising a nucleic acid sequence encoding a polypeptide capable of catalyzing a chemical reaction from a plurality of candidate bacteria comprises the following steps of: (a) generating of a pool of nucleic acids by introducing at least one nucleotide change into the target nucleic acids encoding the polypeptide capable of catalyzing the desired chemical reaction, (b) constructing library vectors to be transformed into a host cell after subcloning said pool of candidate nucleic acids into a surface display vector wherein said resulting vectors direct expression of fusion polypeptides of display motifs and candidate polypeptides and said fusion polypeptides are to be anchored to the surface of said bacteria, (c) transforming said library vectors into bacteria, (d) expressing each of said fusion polypeptides on the surface of host bacteria, and (e) selecting a bacterium expressing a desired polypeptide on the basis of said host bacterial phenotypic changes, or expressing a desired polypeptide on the basis of visual changes of said products.