Searching for Potential Silicon-associated Genes in Cyanobacteria

Jackie Collier1, Mark A Brzezinski2, Stephen B Baines3, Jeffrey W Krause4, Daniel Ohnemus5 and Benjamin S Twining5, (1)Stony Brook University, School of Marine and Atmospheric Sciences, Stony Brook, NY, United States, (2)University of California Santa Barbara, Santa Barbara, CA, United States, (3)Stony Brook University, Stony Brook, NY, United States, (4)Dauphin Island Sea Lab, Dauphin Island, AL, United States, (5)Bigelow Lab for Ocean Sciences, East Boothbay, ME, United States
Abstract:
Recent studies have demonstrated the accumulation of Si in both wild cells and laboratory cultures of marine Synechococcus. Because of their abundance, the cellular Si quotas measured are sufficient to suggest a substantial, unrecognized role for these organisms in the marine Si cycle. Since there is no known role for Si in cyanobacteria, we are using sequenced cyanobacterial genomes to search for pathways of Si metabolism known from other organisms.

Si transporters belonging to four different protein superfamilies have been identified in diverse Si-metabolizing organisms, including diatoms and other protists, plants, bacteria, and sponges. A homolog of ArsB/Lsi2, the arsenite-antimonite efflux porter that can also transport silicate in plants, can be found in many cyanobacteria. However, we have been unable to identify likely influx porter homologs in cyanobacteria, except for predicted proteins with similarity to diatom SIT but only half the length, as well as a few atypical members of the Major Intrinsic Protein (aquaporin) superfamily.

Proteins catalyzing and/or controlling the polymerization of silica have been identified in diatoms and sponges. We have been unable to identify clear homologs of these proteins in cyanobacteria, although cathepsins (belonging to the same protein superfamily as silicateins) are broadly present in cyanobacteria. Proteins that may bind silica in other bacteria (CotB in Bacillus) also lack clear homologs in cyanobacteria. However, since the function of these proteins may depend largely on charge and protein folding characteristics, proteins involved in Si deposition may not be readily identifiable by primary sequence similarity.

The broad diversity of proteins involved in Si metabolism in diverse organisms suggests that each had an independent evolutionary origin. Our results suggest that if Si-associated proteins exist in Synechococcus, they also may have a distinct evolutionary origin unrelated to known Si metabolic pathways.

Back to: Marine Silica Cycle, Silicification, and Silicifiers I