Iodometric method for detection of beta-lactamase activity in yeast cells carrying ampicillin resistance gene in chimeric plasmids

In order to determine whether an ampicillin resistance gene in a chimeric plasmid is active in transformed yeast cells, it is necessary to have a simple and quick assay procedure. We describe here a procedure for achieving this goal using an iodometric color reaction. This method is based on the fact that the ampicillin resistance gene product, beta-lactamase, can hydrolyze penicillin G and release a reducing product, which can be visualized by the discoloration of a dark blue iodine-starch complex. We have improved this method so that the assay can be carried out on agar plate and in liquid culture. It permits the detection of the beta-lactamase enzyme activity in yeast liquid culture at a concentration as low as 1 X 10(5) cells/ml within 12 h. This method is especially useful for certain yeast transformation systems, such as industrial yeast cultures, where the transformants can be selected only by drug resistance.