Cloning of the Thermomonospora fusca Endoglucanase E2 Gene in Streptomyces lividans: Affinity Purification and Functional Domains of the Cloned Gene Product

摘要

Thermomonospora fusca YX grown in the presence of cellulose produces a number of β-1-4-endoglucanases, some of which bind to microcrystalline cellulose. By using a multicopy plasmid, pIJ702, a gene coding for one of these enzymes (E2) was cloned into Streptomyces lividans and then mobilized into both Escherichia coli and Streptomyces albus. The gene was localized to a 1.6-kilobase PvuII-ClaI segment of the originally cloned 3.0-kilobase SstI fragment of Thermomonospora DNA. The culture supernatants of Streptomyces transformants contain a major endoglucanase that cross-reacts with antibody against Thermomonospora cellulase E2 and has the same molecular weight (43,000) as T. fusca E2. This protein binds quickly and tightly to Avicel, from which it can be eluted with guanidine hydrochloride but not with water. It also binds to filter paper but at a slower rate than to Avicel. Several large proteolytic degradation products of this enzyme generated in vivo lose the ability to bind to Avicel and have higher activity on carboxymethyl cellulose than the native enzyme. Other smaller products bind to Avicel but lack activity. A weak cellobiose-binding site not observed in the native enzyme was present in one of the degradation products. In E. coli, the cloned gene produced a cellulase that also binds tightly to Avicel but appeared to be slightly larger than T. fusca E2. The activity of intact E2 from all organisms can be inactivated by Hg2+ ions. Dithiothreitol protected against Hg2+ inactivation and reactivated both unbound and Avicel-bound Hg2+-inhibited E2, but at different rates.