PNNL

Abstract

The a-proteobacterium 'Candidatus Pelagibacter ubique' str. HTCC1062 lacks genes for assimilatory sulfate reduction, making it dependent on organosulfur compounds that occur naturally in seawater. To investigate how these cells acclimate to sulfur limitation, batch cultures were grown in defined media containing either limiting or non-limiting amounts of dimethylsulfoniopropionate (DMSP) as the sole sulfur source. As DMSP initially became limiting, genes located downstream of S-adenosyl methionine (SAM) riboswitches increased up to 50 fold in transcript abundance and up four fold in protein abundance. These included methionine synthesizers (bhmT, mmuM, and metY), an organic hydroperoxidase (osmC), and an oxidoreductase (ordL). After a week in stationary phase, cells limited by sulfur were found to have 6-10 fold greater transcriptional expression of the heme c shuttle ccmC and two small genes of unknown function (SAR11_1163 and SAR11_1164), concomitant with 4.4 fold lower transcript abundance of membrane transporters for DMSP (opuAC) and amino acids (livJ2). These expression profiles suggest that this bacterium's strategy for coping with sulfur stress is primarily intracellular redistribution, rather than increased import of organosulfur.