Drug Dependence and Phenotypic Masking in E. coli

Abstract

STREPTOMYCIN dependent (Sm^D) strains of E. coli L1 (which has an amber mutation in the ornithine transcarbamylase gene which is streptomycin suppressible) will grow when subcultured on Sm agar (500 µg/ml.) or on 3% EtOH agar, but not on paromomycin agar (Pm) or kanamycin (Km) agar, or any mixture of two of the drugs named (Table 1)¹. Paromomycin dependent (Pm^D) mutants selected on Pm agar (200 µg/ml.), however, would grow on subculture to Pm or EtOH agar, EtOH + Pm, EtOH + Km or Pm + Km drug agars; but would not grow on Sm or Km agar, or on any other drug agar which contained streptomycin. These obvious phenotypic differences between Sm^D and Pm^D mutants were eliminated if the strains were first grown on EtOH agar and then subcultured to the different drug agars, when both types of dependent mutant were found to grow on EtOH or Sm or Pm agar, or on EtOH + Pm or Pm + Km agar (Table 1). Gorini et al.¹ considered the latter to be the true phenotype of the strains (which after growth on EtOH agar were called Drug^D) which had previously been masked by Sm or Pm which had so altered the ribosomes as to mask the true properties of the cells. They called the phenomenon “phenotypic masking”.