Characterization of a salt-tolerant family 42 beta-galactosidase from a psychrophilic Antarctic Planococcus isolate

We isolated a gram-positive, halotolerant psychrophile from a hypersaline p ond located on the McMurdo Ice Shelf in Antarctica. A phylogenetic analysis of the 16S rRNA gene sequence of this organism showed that it is a member of the genus Planococcus. This assignment is consistent,vith the morphology and physiological characteristics of the organism, A gene encoding a beta- galactosidase in this isolate was cloned in an Escherichia coli host. Seque nce analysis of this gene placed it in glycosidase family 42 most closely r elated to an enzyme from Bacillus circulans. Even though an increasing numb er of family 42 glycosidase sequences are appearing in databases, little in formation about the biochemical features of these enzymes is available. The refore, we purified and characterized this enzyme. The purified enzyme did not appear to have any metal requirement, had an optimum pH of 6.5 and an o ptimum temperature of activity at 42 degrees C, and was irreversibly inacti vated within 10 min when it was incubated at 55 degrees C. The enzyme had a n apparent K-m of 4.9 mu mol of o-nitrophenyl-beta-D-galactopyranoside and the V-max was 467 mu mol of o-nitrophenol produced/min/mg of protein at 39 degrees C. Of special interest was the finding that the enzyme remained act ive at high salt concentrations, which makes it a possible reporter enzyme for halotolerant and halophilic organisms.