Abstract
A nitrosoguanidine-induced mutant of Escherichia coli K-12 strain JC12 was absolutely dependent on erythromycin or related macrolide antibiotics for growth. The only other drugs which permitted growth (lincomycin and chloramphenicol) are, like the macrolides, inhibitors of the 50S ribosome. The order of relative effectiveness of these drugs was macrolides > lincomycin > chloramphenicol. Rates of growth with all drugs were concentration dependent. Erythromycin starvation was followed by normal rates of increase in cell mass and macromolecular synthesis for approximately one mass-doubling time, after which macromolecular synthesis abruptly ceased and cell lysis and death occurred. The dependent mutant gave rise spontaneously to revertants to independence with very high frequency (10(-4)). The gene (mac) for macrolide dependence is located near minute 25 on the E. coli chromosome; it does not result in increased resistance to these drugs. A separate gene for erythromycin resistance (eryA) is located in the cluster of ribosomal structural genes near spc, close to minute 63. Dependence on macrolides was most clearly evident in strains carrying mutations at both eryA and mac.