THE ESCHERICHIA-COLI FMT GENE, ENCODING METHIONYL-TRANSFER RNAFMET FORMYLTRANSFERASE, ESCAPES METABOLIC CONTROL

The genetic organization near the recently cloned fmt gene, encoding Escherichia coli methionyl-tRNA(f)Met formyltransferase (J. M. Guillon, Y. Mechulam, J. M. Schmitter, S. Blanquet, and G. Fayat, J. Bacteriol. 174:4294-4301, 1992), has been studied. The fmt gene, which starts at a GUG codon, is cotranscribed with another gene, fins, and the transcription start site of this operon has been precisely mapped. Moreover, the nucleotide sequence of a 1,379-bp fragment upstream from fmt reveals two additional open reading frames, in the opposite polarity. In the range of 0.3 to 2 doublings per h, the intracellular methionyl-tRNA(f)Met formyltransferase concentration remains constant, providing, to our knowledge, the first example of a gene component of the protein synthesis apparatus escaping metabolic control. When the gene fusion technique was used for probing, no effect on fmt expression of the concentrations of methionyl-tRNA(f)Met formyltransferase or tRNA(f)Met could be found. The possibility that the fmt gene, the product of which is present in excess to ensure full N acylation of methionyl-tRNA(f)Met, could be expressed in a constitutive manner is discussed.