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Journal of Bacteriology, April 2007, p. 2906-2914, Vol. 189, No. 7
0021-9193/07/$08.00+0     doi:10.1128/JB.01620-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Properties of R-Citramalyl-Coenzyme A Lyase and Its Role in the Autotrophic 3-Hydroxypropionate Cycle of Chloroflexus aurantiacus{triangledown}

Silke Friedmann, Birgit E. Alber, and Georg Fuchs*

Mikrobiologie, Institut Biologie II, Universität Freiburg, Freiburg, Germany

Received 18 October 2006/ Accepted 19 January 2007

The autotrophic CO2 fixation pathway (3-hydroxypropionate cycle) in Chloroflexus aurantiacus results in the fixation of two molecules of bicarbonate into one molecule of glyoxylate. Glyoxylate conversion to the CO2 acceptor molecule acetyl-coenzyme A (CoA) requires condensation with propionyl-CoA (derived from one molecule of acetyl-CoA and one molecule of CO2) to ß-methylmalyl-CoA, which is converted to citramalyl-CoA. Extracts of autotrophically grown cells contained both S- and R-citramalyl-CoA lyase activities, which formed acetyl-CoA and pyruvate. Pyruvate is taken out of the cycle and used for cellular carbon biosynthesis. Both the S- and R-citramalyl-CoA lyases were up-regulated severalfold during autotrophic growth. S-Citramalyl-CoA lyase activity was found to be due to L-malyl-CoA lyase/ß-methylmalyl-CoA lyase. This promiscuous enzyme is involved in the CO2 fixation pathway, forms acetyl-CoA and glyoxylate from L-malyl-CoA, and condenses glyoxylate with propionyl-CoA to ß-methylmalyl-CoA. R-Citramalyl-CoA lyase was further studied. Its putative gene was expressed and the recombinant protein was purified. This new enzyme belongs to the 3-hydroxy-3-methylglutaryl-CoA lyase family and is a homodimer with 34-kDa subunits that was 10-fold stimulated by adding Mg2 or Mn2+ ions and dithioerythritol. The up-regulation under autotrophic conditions suggests that the enzyme functions in the ultimate step of the acetyl-CoA regeneration route in C. aurantiacus. Genes similar to those involved in CO2 fixation in C. aurantiacus, including an R-citramalyl-CoA lyase gene, were found in Roseiflexus sp., suggesting the operation of the 3-hydroxypropionate cycle in this bacterium. Incomplete sets of genes were found in aerobic phototrophic bacteria and in the {gamma}-proteobacterium Congregibacter litoralis. This may indicate that part of the reactions may be involved in a different metabolic process.

* Corresponding author. Mailing address: Mikrobiologie, Institut Biologie II, Schänzlestr. 1, D-79104 Freiburg, Germany. Phone: (49) 761-2032649. Fax: (49) 761-2032626. E-mail: georg.fuchs{at}biologie.uni-freiburg.de.

{triangledown} Published ahead of print on 26 January 2007.

Journal of Bacteriology, April 2007, p. 2906-2914, Vol. 189, No. 7
0021-9193/07/$08.00+0     doi:10.1128/JB.01620-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.



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