EFFECT OF HEME AND OXYGEN AVAILABILITY ON HEMA GENE-EXPRESSION IN ESCHERICHIA-COLI - ROLE OF THE FNR, ARCA, AND HIMA GENE-PRODUCTS

While many organisms synthesize delta-aminolevulinate, the precursor of heme, by condensing succinyl-coenzyme A and glycine, others use a glutamate-dependent pathway in which glutamyl-tRNA dehydrogenase catalyzes the rate-determining step. The hemA gene that encodes this latter enzyme in Escherichia coli has been cloned and sequenced. To examine how its expression is regulated, we constructed hemA-lacZ operon and gene fusions and inserted them into the chromosome in single copy. The effect of aerobic and anaerobic growth conditions and the availability of electron accepters and various carbon substrates were documented. Use of different types of cell culture medium resulted in a fivefold variation in hemA-lacZ expression during aerobic cell growth. Anaerobic growth resulted in 2.5-fold-higher HemA-lacZ expression than aerobic growth. This control is mediated by the fnr and arcA gene products. Fnr functions as a repressor of hem4 transcription during anaerobic cell growth only, whereas the arcA gene product activates hemA gene expression under both aerobic and anaerobic conditions. Integration host factor protein was also shown to be required for control of hemA gene regulation. To determine whether an intermediate or a product of the heme biosynthetic pathway is involved in hem4 regulation, hem4-lacZ expression was analyzed in a hemA mutant. Expression was elevated by 20-fold compared with that in a wild-type strain, while the addition of the heme pathway intermediate delta-aminolevulinate to the culture medium restored expression to wild-type levels. These results suggest that the heme pathway is feedback regulated at the level of hemA gene expression, to supply heme as it is required during different modes of cell growth.