The switch from inorganic to organic sulphur assimilation in Escherichia coli: adenosine 5'-phosphosulphate (APS) as a signalling molecule for sulphate excess.

The utilization of organosulphur compounds as sources of sulphur by Escherichia coli is strongly repressed by sulphate. To search for the signal enabling E. coli to alternate gene expression according to the sulphur source, we investigated the transcriptional control of the ssuEADCB operon, required for the transport and desulphonation of aliphatic sulphonates. We demonstrate that, of the two LysR-type regulators involved in expression from the ssu promoter, Cbl acts as a direct and sufficient activator of transcription in vivo and in vitro, whereas CysB downregulates the promoter efficiency. Most importantly, the Cbl-mediated transcription initiation at the ssu promoter in vitro is abolished in the presence of an early metabolite of the sulphate assimilatory pathway, adenosine 5'-phosphosulphate (APS). This role for APS was confirmed in vivo by measuring the expression of beta-galactosidase from a transcriptional ssu-lacZ fusion in strains containing different mutations blocking the synthesis and consumption of APS. Our data comprise the first evidence that APS may act as the negative cofactor of the transcriptional regulator Cbl, and that APS, and not sulphate itself, serves as the signalling molecule for sulphate excess.