Isolation and nucleotide sequence of the hmp gene that encodes a haemoglobin-like protein in Escherichia coli K-12.

In the course of an attempt to identify genes that encode Escherichia coli dihydropteridine reductase (DHPR) activities, a chromosomal DNA fragment that directs synthesis of two soluble polypeptides of Mr 44000 and 46000 was isolated. These proteins were partially purified and were identified by determination of their N-terminal amino acid sequences. The larger was serine hydroxymethyltransferase, encoded by the glyA gene, while the smaller was the previously described product of an unnamed gene closely linked to glyA, and transcribed in the opposite direction. Soluble extracts of E. coli cells that overproduced the 44 kDa protein had elevated DHPR activity, and were yellow in colour. Their visible absorption spectra were indicative of a CO-binding b-type haemoprotein that is high-spin in the reduced state. The sequence of the N-terminal 139 residues of the protein, deduced from the complete nucleotide sequence of the gene, had extensive homology to almost all of Vitreoscilla haemoglobin. We conclude that E. coli produces a soluble haemoglobin-like protein, the product of the hmp gene (for haemoprotein). Although the protein has DHPR activity, it is distinct from the previously purified E. coli DHPR.