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

Abstract

Summary 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 β‐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.

Sulfur and Nitrogen Limitation in Escherichia coli K-12: Specific Homeostatic Responses

“…Few of these genes are assimilatory. Expression of the scavenging genes is very low on the optimal N source, NH 4 ϩ . Although expression can increase with N limi- tation, it cannot decrease with S limitation.…”

“…Perceiving S limitation involves at least three primary metabolites: sulfide, the reduction product of sulfate, which is used for cysteine biosynthesis; N-acetylserine, a signal that is derived nonenzymatically from O-acetylserine, the only (non-S-containing) precursor of cysteine; and adenosine 5Ј-phosphosulfate (APS), the first intermediate in sulfate assimilation (4,20). There is evidence that the free pool concentrations of sulfide and APS decline under S-limiting conditions and that the pool concentration of N-acetylserine is elevated.…”

“…Both regulators, which are primarily activators, control transcription by the 70 holoenzyme form of RNA polymerase, and both are responsive to ligands. CysB is positively controlled by N-acetylserine and negatively controlled by sulfide or thiosulfate (20), and Cbl is negatively controlled by APS (4). Interestingly, the cbl gene lies just downstream of nac.…”

CbbR, the Master Regulator for Microbial Carbon Dioxide Fixation

“…Many of these amino acid substitutions are centered at the effector pocket, but substitutions in other areas of the LTTR, such as at residues within the linker helix or hinge region or throughout RD-I and RD-II, can generate constitutive activity (70,72,76,86,(88)(89)(90)(91)(92)(93)(94). Typically, single-amino-acid substitutions encompass the vast majority of the reported changes identified for constitutive proteins, and most constitutive proteins bind their effectors but may behave differently from the wild-type LTTR in gel mobility shift assays, DNase I footprinting assays, or in vitro transcription assays (76,88,89,(92)(93)(94). Amino acid substitutions that confer constitutive activity are thought to change the conformation of the LTTR tetramer to mimic the conformation seen when it is bound with the effector or to change the conformation of the LTTR/promoter complex to produce a favorable interaction with RNA polymerase to activate transcription.…”

“…Amino acid substitutions that confer constitutive activity are thought to change the conformation of the LTTR tetramer to mimic the conformation seen when it is bound with the effector or to change the conformation of the LTTR/promoter complex to produce a favorable interaction with RNA polymerase to activate transcription. Studies of LTTR* proteins from several LTTR family members, including NodD, AmpR, OccR, CysB, OxyR, NahR, GtlR, XapR, and Cbl, have been previously published (70)(71)(72)(88)(89)(90)(91)(92)(93)(94)(95)(96).…”