A universal assay for screening expression libraries for carbohydrases

Although many assays are available for the screening of expression librarie s for carbohydrases, some enzymes cannot be detected because their substrat es are incompatible with the existing assays. One thing that all carbohydra ses have in common is that they increase the number of reducing ends when d egrading their substrates. In this paper we explore the possibility of dete cting this increase with the highly sensitive bicinchoninic acid (BCA) redu cing value assay. This assay can be used for the detection of all carbohydr ases degrading any polysaccharide; enzymes with either an exo- or an endo-t ype of mechanism can be detected at the same time. A cDNA library of Asperg illus tubigensis expressed in Kluyveromyces lactis clones, was screened wit h this assay for the presence of xylogalacturonan degrading enzyme(s). High background absorbances caused by culture medium, by proteins produced by t he clones and by substrate could be dealt with by using the precautions des cribed in this note. Three xylogalacturonase producing clones were found us ing this procedure.