Selective enrichments yielded bacterial cultures able to utilize the osmolyte N-methyltaurine as sole source of carbon and energy or as sole source of fixed nitrogen for aerobic growth. Strain MT1, which degraded N-methyltaurine as a sole source of carbon concomitantly with growth, was identified as a strain of Alcaligenes faecalis. Stoichiometric amounts of methylamine, whose identity was confirmed by matrix-assisted, laser-desorption ionization time-of-flight mass spectrometry, and of sulfate were released during growth. Inducible N-methyltaurine dehydrogenase, sulfoacetaldehyde acetyltransferase (Xsc) and a sulfite dehydrogenase could be detected. Taurine dehydrogenase was also present and it was hypothesized that taurine dehydrogenase has a substrate range that includes N-methyltaurine. Partial sequences of a tauY-like gene (encoding the putative large component of taurine dehydrogenase) and an xsc gene were obtained by PCR with degenerate primers. Strain N-MT utilized N-methyltaurine as a sole source of fixed nitrogen for growth and could also utilize the compound as sole source of carbon. This bacterium was identified as a strain of Paracoccus versutus. This organism also expressed inducible (N-methyl)taurine dehydrogenase, Xsc and a sulfite dehydrogenase. The presence of a gene cluster with high identity to a larger cluster from Paracoccus pantotrophus NKNCYSA, which is now known to dissimilate N-methyltaurine via Xsc, allowed most of the overall pathway, including transport and excretion, to be defined. N-Methyltaurine is thus another compound whose catabolism is channelled directly through sulfoacetaldehyde.
INTRODUCTIONTaurine (2-aminoethanesulfonate) is widespread in the biosphere, whether as the free ampholyte or in derivatized form. Mammals usually contain about 8 mM taurine, which they cannot desulfonate (Huxtable, 1992), and which they excrete largely in urine (e.g. Stipanuk, 2004). We presume that this steady supply of taurine to the environment explains the considerable diversity of microbial taurine metabolism, which includes utilizing the compound as a sole source of carbon, nitrogen or sulfur for aerobic growth, as a source of, or as a sink for, electrons for anaerobes, or as a fermentative substrate .One of the few undisputed functions of free taurine in many animals is its role as an osmolyte (Huxtable, 1992;Yancey et al., 2002). One widespread derivative of taurine, Nmethyltaurine (Allen & Garrett, 1971), has now been recognized as an osmolyte in some deep-sea worms, where it can be present at concentrations in excess of 250 mM (Yin et al., 2000). The fate of this compound was unknown until now.The bacterial catabolism of taurine and of the structurally related compounds isethionate (2-hydroxyethanesulfonate), sulfoacetate and ethane-1,2-disulfonate converges at sulfoacetaldehyde (Cook & Denger, 2002). We were interested to see whether N-methyltaurine would be degradable, and if so, whether sulfoacetaldehyde acetyltransferase (Xsc) (EC 2.3.3.15) was involved as well (Fig. 1). Xsc catalyses...