Characterization of the aegA locus of Escherichia coli: control of gene expression in response to anaerobiosis and nitrate

Abstract

Analysis of the DNA sequence upstream of the narQ gene, which encodes the second nitrate-responsive sensor-transmitter protein in Escherichia coli, revealed an open reading frame (ORF) whose product shows a high degree of similarity to a number of iron-sulfur proteins as well as to the beta subunit of glutamate synthase (gltD) of E. coli. This ORF, located at 53.0 min on the E. coli chromosome, is divergently transcribed and is separated by 206 bp from the narQ gene. Because of the small size of the intergenic region, we reasoned that the genes may be of related function and/or regulated in a similar fashion. An aegA-lacZ gene fusion was constructed and examined in vivo; aegA expression was induced 11-fold by anaerobiosis and repressed 5-fold by nitrate. This control was mediated by the fnr, narX, narQ, and narL gene products. Analysis of an aegA mutant indicated that the aegA gene product is not essential for cell respiration or fermentation or for the utilization of ammonium or the amino acids L-alanine, L-arginine, L-glutamic acid, glycine, and DL-serine as sole nitrogen sources. The ORF was designated aegA to reflect that it is an anaerobically expressed gene. The structural properties of the predicted AegA amino acid sequence and the regulation of aegA are discussed with regard to the possible function of aegA in E. coli.