Conserving a volatile metabolite: a role for carboxysome-like organelles in Salmonella enterica

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 degrees 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.

FIG. 1.

Basic ethanolamine metabolism. EA can enter the cell by diffusion, either via EutH or by other, unidentified routes. The diagram depicts the known roles of eut enzymes in metabolism of EA. The loss of acetaldehyde to the gas phase proposed here is indicated by a dotted arrow.

FIG. 2.

Growth and aldehyde release by wild-type cells. Growth (closed symbols) and aldehyde release (open symbols) of strain LT2 grown on minimal acetate (squares), on glycerol (triangles, points up), or on glycerol plus ethanolamine and B₁₂ (triangles, points down). Minimal medium was NCE salts (pH 7.0), which provides ammonia as a nitrogen source. Cultures were grown at 37°C, and ethanolamine was provided at 30 mM. The error bars represent standard deviations.

FIG. 3.

Aldehyde release requires ethanolamine ammonia lyase. Growth (closed symbols) and aldehyde release (open symbols) of LT2 (squares) and a eutB mutant (circles) on minimal glycerol, ethanolamine, and B₁₂ medium. Ethanolamine was provided at 15 mM. The error bars represent standard deviations.

FIG. 4.

Mutations that block acetaldehyde metabolism increase aldehyde release. Mutants eutB (circles), eutD (diamonds), eutE (triangles, points up), and eutG (triangles, points down) were compared to wild-type LT2 (squares) for aldehyde release during growth on glycerol, ethanolamine, and B₁₂. Ethanolamine was provided at 15 mM.

FIG. 5.

Aldehyde release increases in carboxysome mutants. Mutants eutN (circles), eutM (diamonds), eutL (triangles, points up), eutK (right triangles), and eutS (triangles, points down) were compared to wild-type LT2 (squares) for aldehyde release during growth on glycerol, ethanolamine, and B₁₂. Ethanolamine was provided at 15 mM.

FIG. 6.

Effects of other eut mutations on aldehyde release. Mutants eutH (circles), eutJ (diamonds), eutQ (triangles, points up), eutT (right triangles), and eutP (triangles, points down) were compared to wild-type LT2 (squares) for aldehyde release during growth on glycerol, ethanolamine, and B₁₂. Ethanolamine was provided at 15 mM.

FIG. 7.

Aldehyde release during growth on ethanolamine as a carbon source; growth (closed symbols) and aggregated aldehyde release normalized to cell growth (open symbols). Wild-type LT2 (squares) was compared to a eutM deletion mutant (triangles, points down). Ethanolamine (40 mM) was the sole carbon source. The error bars represent standard deviations.

FIG. 8.

Effect of pH on aldehyde release. Growth (closed symbols) and cumulative aldehyde release (open symbols) were measured at pH 7 and 8. Wild-type LT2 is depicted by squares at pH 7 and diamonds at pH 8. A eutM deletion mutant is depicted at pH 7 (triangles, points down) and pH 8 (triangles, points up). The medium was buffered with 50 mM MOPS (pH 7) or Bicine (pH 8) and contained glycerol, ethanolamine (20 mM), and B₁₂. The error bars represent standard deviations.

FIG. 9.

Aldehyde release by pdu mutants. Mutants pduCDE (circles), pduP (triangles, points down), pduB (diamonds), pduJ (triangles, points up), and pduK (right triangles) were compared to wild-type LT2 (squares) during growth on pyruvate (40 mM), 1,2-propanediol (80 mM), and B₁₂. The cultures were buffered at 7.0 with NCE salts and incubated at 38°C. All strains grew at the same rate (data not shown).

FIG. 10.

Proposed role of the carboxysome. It is proposed here that the primary function of a carboxysome is to retain and prevent loss of a volatile metabolite, acetaldehyde in the case of the ethanolamine pathway. Retention of volatile aldehydes (in Salmonella) and retention of CO₂ (in other organisms) might be explained if the carboxysome provided a low-pH compartment.