Transcriptional control of the cysG gene of Escherichia coli K‐12 during aerobic and anaerobic growth

Abstract

The 74-min region of the Escherichiu coli chromosome includes five open reading frames of known sequence. The first and last of these genes, nirB and cysG, are transcribed in the same direction and both are essential for NADH-dependent nitrite reductase activity. The functions of the other genes, nirD, nirE and nirC, which are located between nirB and cysG, are unknown. The nirB gene is transcribed from a promoter which is anaerobically induced, expression being dependent on the transcription activator protein, Fnr. Here we show that the nirD, nirE, nirC and cysG genes are also expressed from the nirB promoter. After subcloning cysG, a second promoter was located less than 100 bases upstream of cysG. Two groups of transcription start points separated by 40 bases were detected in this region by S1 mapping. Rates of transcription from the isolated cysG promoter were the same during aerobic growth and anaerobic growth in the presence or absence of nitrite. However, when the nirB gene and its promoter were cloned back upstream from the cysG promoter, the rate of transcription was higher during anaerobic growth than during aerobic growth and was further induced by nitrite. These increases were totally dependent on a functiona1,fnr gene and were shown by S1 mapping experiments to be due to transcriptional readthrough from the Fnr-dependent nirB promoter.No promoter activity was associated with DNA fragments between the BumHI site located within the Nterminal coding region of the nirB gene and the cysG promoter located at the C-terminus of nirC.Sirohaem is one of the prosthetic groups of both the NADH-dependent nitrite reductase and the NADPH-dependent sulphite reductase of Escherichiu coli [I]. Sulphite reductase is an essential enzyme in the synthesis of cysteine from sulphate. Mutants defective in the cysG gene are unable to grow without cysteine either aerobically or anaerobically because the cysG product is essential to convert uroporphyrinogen 111 into sirohaem. As the NADH-dependent nitrite reductase is the most active enzyme for reducing nitrite to ammonia in E. coli, cysG mutants are also defective in nitrite reduction [2].The cysG gene is located extremely close to nirB, the structural gene for the nitrite reductase apoprotein, but is separated from it by three open reading frames which, in the preceding paper, we have designated nirD, nirE and nirC [3]. The third of these open reading frames, nirC, encodes a 28.5-kDa polypeptide with regions of amino acid sequence similar to cytochrome oxidase polypeptide 1 [3].The nirB gene is not expressed during aerobic growth. Transcription of nirB during anaerobic growth is dependent upon a functional Fnr protein and is further induced by nitrite (4, 51. As nirB and cysG are transcribed in the same direction (from nirB towards cysG), the aims of the experiments deCorrespondence to J. A. Cole,

Genetic structure and regulation of the cysG gene in Salmonella typhimurium

“…Previous work on E. coli and S. typhimurium has suggested that the cysG gene is in an operon with and downstream of the structural gene for nitrite reductase, nirB (31,32,41). Our data agree with that conclusion.…”

“…Transcription of the cysG gene was induced in response to nitrite when cells were grown under anaerobic conditions. Molecular analysis of the cysG gene in E. coli suggests that the gene is in an operon with the nirB gene, which encodes the structural gene for the NADH-dependent nitrite reductase (25,31,32). Sequence analysis of the cysG gene suggests that this is also the case in S. typhimurium (41).…”

Nucleotide sequence, organisation and structural analysis of the products of genes in the nirB–cysG region of the Escherichia coli K‐12 chromosome

“…1. Although the function, if any, of the central three open reading frames is unknown, we shall show in the following paper [18] that transcripts starting at nirB cross nirD,E,C into cysG. They have therefore been designated nirD, nirE and nirC.…”

“…2). Although nirC is transcribed under the control of the nirB promoter [18], far less NirC than NirB protein accumulates in the bacteria (unpublished results). Decreased expression of nirC relative to nirB could be due to transcription termination between the two genes, to a weaker ribosome binding site preceding nirC, or to the unfavorable pattern ofcodon usage in the N-terminal region of the protein.…”

The N-terminal Domain of Escherichia coli Assimilatory NADPH-Sulfite Reductase Hemoprotein Is an Oligomerization Domain That Mediates Holoenzyme Assembly

“…Transcription of the cysJIH region of the operon is enhanced in response to N-acetyl-L-serine, the cysteine precursor, by the LysR-like transcription factor CysB (60). Despite its name, siroheme synthase (CysG) is part of the nitrate regulon, expressed constitutively at low levels but not overexpressed in response to sulfite starvation (61,62). Transcription of the nirBDCG-cysG region of the operon is enhanced in response to NarL and NarP, which sense nitrite/ nitrate, or the Fnr transcription factor, which senses oxygen (63).…”