Phenotypic Suppression by Streptomycin of Amber Mutants in the Ribonucleic Acid Bacteriophage Coat Protein Cistron

After mutagenesis with nitrosoguanidine or ultraviolet light, 298 streptomycin high-resistant and 98 streptomycin high-dependent mutants were isolated from HfrC Su − . They were tested for their ability to phenotypically suppress five different amber ribonucleic acid (RNA) bacteriophage mutants in the presence of streptomycin. The phage mutants are all in the coat protein, which is 129 amino acids long; the uracil-adenine-guanine codons were at the following positions: sus 3 and am B2, 6; am B11, 50; am B21, 54; sus 11, 70. Only sus 3 and am B2 could be phenotypically suppressed by streptomycin; this was clearly demonstrated in nine mutant strains, seven str -HR and two str -HD. The suppression was always dependent upon added streptomycin and was dose-dependent in all cases. None of the mutants showed measurable suppression in absence of the drug. Among revertants to streptomycin independence from streptomycin-dependent strains that could show phenotypic suppression, most of those that were still resistant to streptomycin (10 μg or more) retained the capacity to show phenotypic suppression; whereas among those revertants sensitive to 10 μg of streptomycin or less, none retained the capacity. Eight different amber polar mutants (strong and weak) in gene 34 of phage T4 were also tested for pleiotypic suppression by streptomycin in all the streptomycin-resistant and -dependent strains isolated. No suppression was found in any of the 396 strains tested.

Conversion by pregnenolone to progesterone by rabbit placenta in vitro

A placental preparation from rabbits in mid-pregnancy was shown to convert 7α-3H-pregnenolone to radioactive progesterone in vitro. The extent of conversion per gram tissue by the placenta was about one hundredth of that by the ovary from the pregnant rabbit. Identification of radioactive progesterone was accomplished by reverse isotope dilution and recrystallization to constant specific activity. However, no Δ5-3β-hydroxysteroid dehydrogenase was found histochemically in the trophoblast and other tissues of the rabbit placenta. No ability to convert 7α-3H-pregnenolone to radioactive 20α-hydroxypregn-4-en-3-one, 20β-hydroxypregn-4-en-3-one, corticosterone, cortisol, androst-4-ene-3,17-dione, dehydroepiandrosterone, 17β-oestradiol and oestrone in placental preparation from rabbits could be demonstrated.

The data demonstrate the presence of a 3β-ol-dehydrogenase enzyme system in the rabbit placenta.

Loss of lysis inhibition in filamentous Escherichia coli infected with wild-type bacteriophage T4

The plaque enlargement of wild-type T4 bacteriophage observed when assayed in the presence of low concentrations of mitomycin C or after exposure to very low doses of ultraviolet light was studied by using solid as well as liquid culture media. It was found that the filamentous cell formed by the treatment with the agents is responsible for the phenomenon. The filamentous cell was also shown to be characterized not only by the loss of capacity of lysis inhibition but also by a shortening of the latent period. No difference in cellular rigidity could be seen between the filamentous cell and normal cell as far as the analysis from the outside of the cell was concerned, whereas the former cell was shown to be more readily susceptible to phage-induced lysozyme from the inside of the cell. A possible change in the membrane of the filamentous cell and a possible mechanism for lysis inhibition are discussed.

Mutations to spectinomycin resistance which alleviate the restriction of an amber suppressor by streptomycin resistance

Unlike mutations to streptomycin resistance, which restrict the function of an SU II amber suppressor, mutations to spectinomycin resistance have no restrictive effect. Furthermore, mutations to spectinomycin resistance counteract the restrictive action of streptomycin-resistant mutations when combined in a single strain. The mutations to drug resistance, singly or in combination, do not themselves lead to any detectable suppressor activity. The effects on SU II are interpreted in terms of interactions between ribosomal elements.