Spontaneously arising mutants, designated trpER, result in the loss of trpE gene function, and simultaneously confer partial constitutivity on the trp operon of E. coli. The mutational defect in the ER strain has been characterized as a single, small alteration; transduction crosses with deletion and point mutants place the altered site in the operator-distal region of the trpE gene. On the basis of genetic analyses, it appears that the remainder of the genetic material of the trp operon of the ER strain is unaltered, since the regions responsible for normal operator, promoter and the other structural genes of the operon can be separated from the ER strain. Furthermore, the capacity for normal functioning of these elements in messenger RNA and enzyme synthesis is demonstrable in the ER strain. The ER mutant reverts spontaneously to prototrophy (frequency ~ 2 × 10-8), but reversion is not induced by 2-aminopurine, nitrosoguanidine, ethylmethane sulphonate or ICR. Reversion to prototrophy has always been observed to result in loss of the constitutivity of ER. No effect on the properties of ER are observed in the presence of suppressors of the nonsense codons UAG, UAA, or UGA. The constitutivity of ER is cis-dominant, and is not influenced by the introduction of a strong polar mutation preceding it in the E gene; thus constitutivity is not mediated by a nascent or released altered product of the E gene. Constitutivity is clearly the result of the internal initiation of transcription at or near the ER altered site, as proved by hybridization studies of trp messenger RNA synthesis. This point is confirmed by the lack of dependence of expression upon derepression of the operator, and in part by the indifference of constitutivity to the presence of a nonsense mutation within the operator-proximal region of the E gene. Initiation at the ER locus alone (in an operator-repressed cell) shows an efficiency which is 30 to 50% of the maximum value observed for operator-controlled initiations (in a trpR- cell under steady-state conditions). No repression of the ER initiations was detected ha cells grown in very rich medium. © 1969.
