Sulfonate-sulfur metabolism and its regulation in Escherichia coli

In the absence of sulfate and cysteine, Escherichia coli can use aliphatic sulfonates as a source of sulfur for growth. Starvation for sulfate leads to the expression of the tauABCD and ssuEADCB genes. Each of these gene clusters encodes an ABC-type transport system required for uptake of aliphatic sulfonates and a desulfonation enzyme. The TauD protein is an α-ketoglutarate-dependent dioxygenase that preferentially liberates sulfite from taurine (2-aminoethanesulfonic acid). SsuD is a monooxygenase that catalyzes the oxygenolytic desulfonation of a range of aliphatic sulfonates other than taurine. Its cosubstrate is FMNH2, which is provided by SsuE, an NAD(P)H-dependent FMN reductase. In contrast to many other bacteria, E. coli is unable to grow with arylsulfonates or with sulfate esters as sulfur source. The tau and ssu systems thus provide all genes for the utilization of known organosulfur sources by this organism, except the as yet unidentified gene(s) that enable some E. coli strains to grow with methanesulfonate or cysteate as a sulfur source. Expression of the tau and ssu genes requires the LysR-type transcriptional regulatory proteins CysB and Cbl. Synthesis of Cbl itself is under control of the CysB protein, and the CysB protein may therefore be regarded as the master regulator for sulfur assimilation in E. coli, while the Cbl protein functions as an accessory element specific for utilization of sulfur from organosulfur sources.