β-Lactamase Reporter System for Selecting High-Producing Yeast Clones

In modern production of protein biopharmaceuticals, a good screening and selection method of high-producing clones can dramatically influence the whole production process and lead to lower production costs. We have created a rapid, simple, and inexpensive method for selecting high-producing clones in the yeast Pichia pastoris that is based on the beta-lactamase reporter system. By integrating the reporter gene and the gene of interest into the same genome locus, it was possible to use beta-lactamase activity as a measure of the expression level of the protein of interest. A novel expression vector with two independent expression cassettes was designed and tested using green fluorescent protein (GFP) as a model. The first cassette contained the GFP gene under the control of a strong, inducible AOX1 promoter, while the second cassette consisted of the beta-lactamase reporter gene under the control of a weak constitutive YPT1 promotor. High-producing GFP clones were selected directly on the plates based on the color change after hydrolysis of the beta-lactamase substrate added to the medium. beta-lactamase activity was found to positively correlate with GFP fluorescence. The reporter system described is widely applicable-it can be easily applied to other, also pharmaceutically relevant proteins and to other yeast expression systems, such as Saccharomyces cerevisiae and Hansenula polymorpha.