THE EFFECT OF ACRIDINE DYES ON MATING TYPE FACTORS IN ESCHERICHIA COLI

Abstract

In Escherichia coli, female strains are designated as F-; male strains are of two kinds, designated F+ and Hfr, respectively. The male determinant in Hfr strains behaves as a chromosomal factor allelic to F-.1-3 In F+ strains, however, maleness is determined by a factor "F," with remarkable properties, notably its easy, con-tagious transmission to F-cells.4-6 Cells carrying F can be disinfected by treat-ments with cobalt ion and with acridine dyes.7. I These properties support the con-clusion that F is a plasmid, an extrachromosomal particle, which is readily trans-ferred during mating contacts. It has been suggested that Hfr strains represent the incorporation of F as an element of the chromosome. 1, 2, 4 5 9 Jacob and Woll-man introduced the term "episome" for a plasmid that has a facultative association with the chromosome.9 The present paper reports further evidence for this con-ception, namely on the mechanism by which F is eliminated by the acridine dyes. Materials and Methods.-Two acridine dyes, proflavine (PF, 2: 8-diamino-acridine), and acridine orange (AO, 2:8-bisDimethylaminoacridine) were used. Stock solutions containing 100 ,ug per ml of PF or 500 ,g per ml of AO in water were autoclaved and stored in the dark for periods up to a week. EM-sugar agar"' was used as a selective medium. To grow on this medium, a recombinant must be prototrophic and also be able to ferment the sugar, e.g., lac-tose. Nutrient medium used for acridine treatment consisted of Difco peptone, 10 and Difco meat extract, 10 gm per liter. The pH of the medium was adjusted with sodium hydroxide solution using the Beckman pH meter. Difco penassay broth (Antibiotic assay medium number 3) was used routinely for bacteriological work. Strains of E. coli used in these experiments are mutants derived from strain K-12. The production, origin, and characteristics of these mutants are summarized in Table I. The strain used for acridine treatment was mainly W6. Recombination technique: Overnight cultures of tester strains are streaked on EM-sugar medium and they are cross-brushed against one loopful of the culture being tested. Recombinants arise at the junction of the two cultures only in com-patible combinations.4 Acridine method: An overnight F+ culture is diluted to 104 cells per ml in a