Phylogenetic analyses of eukaryotic SCP-x thiolase domains reveal that they are related to putative thiolases encoded in proteobacterial genomes (Peretóet al., 2005). Based on the phenotype of the skt-mutant strains G12 and Chol1-KO[skt] and on the similarities to the SCP-x thiolase domain, we conclude that the gene skt encodes a β-ketothiolase that catalyzes the thiolytic release of acetyl-CoA from the CoA-ester of the so far presumptive 7,12-dihydroxy-3,22-dioxo-1,4-diene-24-oate (V). The reaction products would be DHOPDC-CoA (VI), which has been detected in cell extracts of strain Chol1

previously (Birkenmaier et al., 2007), and acetyl-CoA. As the gene product of skt and its orthologs in the other cholate-degrading bacteria mainly show similarities to the SCP-x thiolase domain only and not to the SCP-2 domain of SCP-x, the annotation of these putative proteins as nonspecific lipid transfer proteins CDK inhibitor is misleading. However, Skt and its orthologs have a highly conserved motif at their C-terminus that is very similar PF-562271 to two short motifs

within the sterol-binding SCP-2 domain of the human SCP-x (Fig. 2), suggesting that this region of the bacterial proteins might be involved in interacting with the steroid skeleton of cholate. Regarding the function of Skt, it appeared surprising that DHOCTO was the major accumulating product because one would rather expect 7,12-dihydroxy-3,22-dioxo-1,4-diene-24-oate (DHDODO), the presumptive hydrolysis product of CoA-ester V, to accumulate as a dead-end metabolite. DHDODO is a β-ketoacid, which is prone to spontaneous decarboxylation. However,

we did not detect DHDODO or a presumptive decarboxylation product in our analyses. Thus, the fact that DHOCTO was the major accumulating compound suggests that blocking β-oxidation at the last step causes a negative feedback inhibition on the previous enzymatic steps. As a consequence, the CoA-esters of DHOCTO and THOCDO are hydrolyzed and the free bile salts are released. In our earlier study on the transposon mutant strain R1, we had never detected DHOCTO or THOCDO in culture supernatants (Birkenmaier et al., 2007). This indicates that the conversion of Δ1,4-3-ketocholyl-CoA (II) to DHOPDC-CoA (VI) may proceed in a tightly controlled canalized process without Orotic acid a significant release of degradation intermediates. In agreement with this hypothesis, it is also believed that β-oxidation of fatty acids occurs by substrate channelling in multienzyme complexes (Kunau et al., 1995; Peretóet al., 2005). Our study is a further step towards the verification of the pathway for the β-oxidation of the acyl side chain of cholate by strain Chol1. To elucidate this reaction sequence further, biochemical investigations regarding the formation and metabolism of the respective CoA-esters of DHOCTO and THOCDO are under way in our laboratory. We have now identified two genes, acad and skt, that encode proteins required for this part of cholate degradation.