The 17-gene ethanolamine (eut) operon of Salmonella typhimurium encodes five homologues of carboxysome shell proteins - PubMed (original) (raw)
The 17-gene ethanolamine (eut) operon of Salmonella typhimurium encodes five homologues of carboxysome shell proteins
E Kofoid et al. J Bacteriol. 1999 Sep.
Abstract
The eut operon of Salmonella typhimurium encodes proteins involved in the cobalamin-dependent degradation of ethanolamine. Previous genetic analysis revealed six eut genes that are needed for aerobic use of ethanolamine; one (eutR), encodes a positive regulator which mediates induction of the operon by vitamin B12 plus ethanolamine. The DNA sequence of the eut operon included 17 genes, suggesting a more complex pathway than that revealed genetically. We have correlated an open reading frame in the sequence with each of the previously identified genes. Nonpolar insertion and deletion mutations made with the Tn10-derived transposable element T-POP showed that at least 10 of the 11 previously undetected eut genes have no Eut phenotype under the conditions tested. Of the dispensable eut genes, five encode apparent homologues of proteins that serve (in other organisms) as shell proteins of the carboxysome. This bacterial organelle, found in photosynthetic and sulfur-oxidizing bacteria, may contribute to CO2 fixation by concentrating CO2 and excluding oxygen. The presence of these homologues in the eut operon of Salmonella suggests that CO2 fixation may be a feature of ethanolamine catabolism in Salmonella.
Figures
FIG. 1
Suggested pathway for metabolism of ethanolamine. Gene assignments are based on direct assays (EutBCE and CobA), on mutant phenotypes (EutA, EutT, Ack, Pta, and glyoxylate shunt), or on sequence similarity (EutG). The homologues of carboxysome shell proteins suggest the possibility of CO2 fixation, which has not been demonstrated. In the diagram, the outer boundary is the cell membrane; the role of carboxysomes in this pathway is unknown.
FIG. 2
Diagram of the eut operon sequence. The genes underlined in black were discovered by genetic characterization of mutants defective for aerobic use of ethanolamine. The regions underlined in gray were sequenced previously (27, 60). The transposon shown in the E. coli sequence was found in one isolate of E. coli (8) but not in another (69).
FIG. 3
Alignment of carboxysome shell protein homologues. The top panel shows the alignment of the EutN protein with the CcmL protein of Synechococcus. The middle panel aligns the EutK and EutM proteins with the CcmK protein of Synechococcus and the PduA protein of the Salmonella pdu (propanediol utilization) operon. The bottom panel aligns the EutL and EutS proteins with the PduB protein from the Salmonella pdu operon. The EutLS family is most similar to the PduA protein of the EutK, EutN, CcmK class. The sequence features shared by the EutLS-PduB family and the PduA protein are indicated by the black dots below the sequences.
FIG. 4
Sequence motif that EutJ and EutA proteins share with the chaperonin DnaK. Only EutJ shows significant similarity to DnaK over its entire length; EutJ and EutA are not significantly similar to each other, but they share the motif mentioned above. The central DIGGT motif is part of a nucleotide binding loop in the DnaK protein (55).
References
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