The origin of His+ revertants of Salmonella typhimurium obtained on selective medium

The spontaneous reversion to His+ of the Salmonella typhimurium alleles hisD3052 and hisG46 was investigated. In fluctuation tests, the expected "jackpot" distribution of His+ revertants was not observed. The experimental distributions were close to Poisson distribution. The redistribution test showed no significant differences in the His+ colony counts between spread and unspread plates. An attempt at indirect selection of His+ revertants in fluid medium failed. It was also shown that the mean number of His+ reversion events and the mean number of revertants per plate were similar. At the same time, kanamycin-resistant mutants had jackpot distribution. Selection for His+ revertants (histidine starvation) did not increase mutation to Kanr.