The photoreaction center of Rhodobacter sphaeroides: a ‘biosensor protein’ for the determination of photosystem-II herbicides?

According to a European directive for drinking water 1980, pesticides are tolerable only in extremely low concentration of 0.1 μg 1−1 Hence, there is a demand for the sensitive control of pesticides in drinking water. Currently used detection methods (LC and GC-MS) are not suitable for rapid screening, but progress has been made recently to apply immunoassays based on pesticide-specific antibodies. Here, we present an alternative receptor, photosystem-II (PSII), which interacts with photosystem-II herbicides. In particular, we review the structure and function of the photosynthetic reaction center (RC) from Rhodobacter sphaeroides and its application for the detection of PSII herbicides using different biosensor formats. An assay based on photobleaching of RC, a displacement assay based on bacterial luciferase as detection system and a grating coupler device for measuring the amount of PSII herbicides is discussed. While these assays work in principle, they are not sensitive enough to comply to the detection limits set by EU regulation. However, both the sensitivity and selectivity of RC might be increased by modification of the RC using genetic engineering techniques.