Regulation of heme biosynthesis in Escherichia coli

Escherichia coli is an organism that synthesizes 5-aminolevulinate (ALA), the first committed compound of the heme biosynthetic pathway, from glutamate (C-5 pathway) as opposed to glycine and succinyl CoA (C-4 pathway). While regulation of the C-4 pathway is generally acknowledged to occur at the level of formation of ALA, the mode of regulation of the C-5 pathway is currently unclear. Here we have examined one aspect of regulation of heme synthesis in E. coli: the role of the end product, heme, as a feed-back regulator of ALA production. By using plasmid-encoded ALA synthase and/or cytochrome b5 expressed in a wild type E. coli strain, it was possible to determine the role that the proposed regulatory heme pool plays in the regulation of ALA and heme production. Expression of rat-soluble cytochrome b5 results in an increase of cellular heme, indicating that the cell responds to this foreign "heme sink" by producing more heme even though the cytochrome does not participate directly in normal cellular regulation. Accumulation of pathway intermediates does not occur under these conditions. Expression of plasmid-encoded mouse ALA synthase results in increased cellular heme production as well as the accumulation of pathway intermediates either in the presence or absence of plasmid encoded cytochrome b5. These data support a regulatory scheme where the heme biosynthetic pathway in this C-5 organism is regulated at the level of ALA production in part by cellular heme content.