Mutations creating a new initiation point for expression of the histidine operon in Salmonella typhimurium

Abstract

An operator normally controls expression of the histidine operon in Salmonella typhimurium . Deletion of this operator and a portion of the adjacent G gene in mutant hisG203 results in loss of expression of the entire operon. Mutant G203 does not revert to prototrophy, either spontaneously or following mutagen treatment. However, secondary mutants can be selected for their ability to grow on histidinol, i.e. for restored expression of the intact D gene. Of 145 secondary mutants isolated, at least 91 contain point mutations mapping in the G gene. Seventy-nine of these secondary mutants have been found to be identical; another twelve mutants are probably also the same. Each of the 79 point mutations maps in region VI of G and appears to be allelic with the type point mutant, 1306 . The secondary point mutations, in the G203 background, restore about 50% of the D enzymic activity found in wild-type repressed cells. Histidine repression control, absent in G203 secondary mutants, is restored when G203 is transduced to G203 + . The mutations, in the G203 + background, elicit a feedback hypersensitive G enzyme; this in turn leads to a cold sensitive phenotype. At 37°C the mutants grow like wild-type bacteria; at 20°C they require histidine or histidinol for growth. The experimental evidence indicates that the site of the secondary mutation is critical for expression of the operon in G203 . This site may, by base pair transition, become an initiator for transcription or for translation of messenger RNA.