In other words:
• They deleted an enzyme that previous work showed could likely be replaced.
• They added the necessary nutrient histidine because previous work showed that mutations conferring an ability to make tryptophan destroyed the ability to make histidine.
• The added histidine would have shut off production of the protein, so they removed the genetic control element to keep it in production.
• Later, once they found mutations to produce tryptophan, they removed histidine from the medium to encourage the production of mutations restoring histidine synthesis.
Roll the ball to the left to avoid one obstacle, roll it backward to avoid another, turn the maze over to drop the ball into the next corridor. . . . Needless to say, this ain't how unaided nature works -- unless nature is guiding events toward a goal.