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 o f heme, by condensing succinyl-coenzyme A and glycine, others use a gl utamate-dependent pathway in which glutamyl-tRNA dehydrogenase catalyz es the rate-determining step. The hemA gene that encodes this latter e nzyme in Escherichia coli has been cloned and sequenced. To examine ho w its expression is regulated, we constructed hemA-lacZ operon and gen e fusions and inserted them into the chromosome in single copy. The ef fect of aerobic and anaerobic growth conditions and the availability o f electron accepters and various carbon substrates were documented. Us e of different types of cell culture medium resulted in a fivefold var iation in hemA-lacZ expression during aerobic cell growth. Anaerobic g rowth resulted in 2.5-fold-higher HemA-lacZ expression than aerobic gr owth. This control is mediated by the fnr and arcA gene products. Fnr functions as a repressor of hem4 transcription during anaerobic cell g rowth only, whereas the arcA gene product activates hemA gene expressi on under both aerobic and anaerobic conditions. Integration host facto r protein was also shown to be required for control of hemA gene regul ation. To determine whether an intermediate or a product of the heme b iosynthetic pathway is involved in hem4 regulation, hem4-lacZ expressi on was analyzed in a hemA mutant. Expression was elevated by 20-fold c ompared with that in a wild-type strain, while the addition of the hem e pathway intermediate delta-aminolevulinate to the culture medium res tored expression to wild-type levels. These results suggest that the h eme pathway is feedback regulated at the level of hemA gene expression , to supply heme as it is required during different modes of cell grow th.