Secreted beta-galactosidase from a Flavobacterium sp. isolated from a low-temperature environment

The bacterial strain Flavobacterium sp. 4214 isolated from Greenland was found to express beta-galactosidase (EC 3.2.1.23) at temperatures below 25 degrees C. A chromosomal library of Flavobacterium sp. 4214 was constructed in Escherichia coli, and the gene gal4214-1 encoding a beta-galactosidase of 1,046 amino acids (114.3 kDa) belonging to glycosyl hydrolase family 2 was isolated. This was the only gene encoding beta-galactosidase activity that was identified in the chromosomal library. Expression levels in both Flavobacterium sp. 4214 and in initial recombinant E. coli strains were insufficient for biochemical characterization. However, a combination of T7 promoter expression and introduction of an E. coli host that complemented rare transfer RNA genes yielded 15 mg of beta-galactosidase per liter of culture. Gal4214-1-His protein was found to be active in monomeric conformation. The protein was secreted from the cytoplasm, probably through an N-terminal signaling sequence. The Gal4214-1-His protein was found to have optimum activity at a temperature of 42 degrees C, but with short-term stability at temperatures above 25 degrees C.