Characterizing proteases in an Antarctic Janthinobacterium sp. isolate： Evidence of a protease horizontal gene transfer event

摘要

We report the isolation of a cold-adapted bacterium belonging to the genus Janthinobacterium(named AU11), from a water sample collected in Lake Uruguay(King George Island, South Shetlands). AU11(growth between 4℃ and 30℃) produces a single cold-active extracellular protease(Ex PAU11), differentially expressed at low temperature. Ex PAU11 was identified by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry(MALDI-To F MS) as an alkaline metallo-protease(70% coverage with an extracellular protease of Janthinobacterium sp. PI12), and by protease-inhibitor screening identified as a serine-protease. To the best of our knowledge this is the first experimental evidence of a cold-active extracellular protease produced by Janthinobacterium. Furthermore, we identified a serine-protease gene(named JSP8A) showing 60% identity(98% query coverage) to subtilisin peptidases belonging to the S8 family(S8A subfamily) of many cyanobacteria. A phylogenetic analysis of the JSP8 A protease, along with related bacterial protein sequences, confirms that JSP8 A clusters with S8 A subtilisin sequences from different cyanobacteria, and is clearly separated from S8 A bacterial sequences of other phyla(including its own). An analysis of the genomic organization around JSP8 A suggests that this protease gene was acquired in an event that duplicated a racemase gene involved in transforming L- to D-amino acids. Our results suggest that AU11 probably acquired this subtilisin-like protease gene by horizontal gene transfer(HGT) from a cyanobacterium. We discuss the relevance of a bacterial protease-HGT in the Antarctic environment in light of this hypothesis.