A heme-deficient strain of Escherichia coli has a three-base pair deletion in a "hotspot" in hemA.

The key regulatory step in heme biosynthesis in Escherichia coli is at the level of glutamyl-tRNA reductase (GTR), an enzyme which is encoded by hemA. A strain, HU227, with a spontaneous in-frame mutation in hemA has no GTR activity. The mutation is shown to be a three-base deletion at a "hotspot" in the gene.