Conserving a Volatile Metabolite: a Role for Carboxysome-Like Organelles in Salmonella enterica

Abstract

ABSTRACT Salmonellae can use ethanolamine (EA) as a sole source of carbon and nitrogen. This ability is encoded by an operon ( eut ) containing 17 genes, only 6 of which are required under standard conditions (37°C; pH 7.0). Five of the extra genes ( eutM , - N , - L , - K , and - G ) become necessary under conditions that favor loss of the volatile intermediate, acetaldehyde, which escapes as a gas during growth on EA and is lost at a higher rate from these mutants. The eutM , -N , -L , and - K genes encode homologues of shell proteins of the carboxysome, an organelle shown (in other organisms) to concentrate CO 2 . We propose that carboxysome-like organelles help bacteria conserve certain volatile metabolites—CO 2 or acetaldehyde—perhaps by providing a low-pH compartment. The EutG enzyme converts acetaldehyde to ethanol, which may improve carbon retention by forming acetals; alternatively, EutG may recycle NADH within the carboxysome.