Activation-independent capture of free fatty acids at the bacterial cell envelope by BrtB

Fatty acids (FAs) are key metabolites in living organisms, shaping membrane architecture, and helping cellular acclimatization to changing conditions. Bacteria synthesize FAs de novo but can also reclaim them from membrane lipids or uptake them from the environment, usually converting free fatty acids (FFAs) into activated FAs that can then be further metabolized. In cyanobacteria, it is the acyl-acyl carrier protein synthetase (Aas) that activates exogenous FFAs. However, the cyanobacterial enzyme BrtB was recently shown to esterify, in vitro, FFAs directly onto abundant chlorinated glycolipids (bartolosides), generating bartoloside fatty acid esters (B-FAs). Whether this chemistry operates in vivo, where it occurs, and what its implications are for cell physiology have remained unclear. Here, we show that in the cyanobacterium Synechocystis salina LEGE 06099, BrtB likely esterifies exogenous FFAs at or near the cell envelope without prior activation to generate B-FAs. We found that supplemented FFAs were converted into B-FAs within minutes. Unexpectedly, this response occurred with minimal changes in gene expression and little alteration of the extracellular proteome, consistent with a pathway already in place. Additionally, we observed that B-FAs can further be hydrolyzed into hydroxybartolosides, the levels of which increase in response to FA supply, suggesting a transient sequestration of FFA. These findings identify a specific activation-independent route for the incorporation of exogenous FFAs into cyanobacterial specialized metabolites, expanding the known repertoire of bacterial FA utilization.

Fatty acids (FAs) are essential building blocks of cell membranes and key metabolites in all living organisms. While bacteria synthesize FAs de novo, they can also incorporate exogenous FAs, saving the energetic cost of biosynthesis. Bacteria are generally thought to require their prior activation before they can be incorporated into metabolism. Here we show that a marine cyanobacterium bypasses this requirement through BrtB, which directly esterifies exogenous FAs onto specialized glycolipids in vivo without prior activation, likely at or near the cell envelope. This process occurs within minutes and is accompanied by minimal changes in gene expression, consistent with a pre-existing pathway. Our findings identify a specific activation-independent route for incorporation of exogenous FAs into cyanobacterial specialized metabolites and suggest that these glycolipids may transiently sequester FAs in response to environmental availability.