ABSTRACT
Continuing the genetic and biochemical characterization of the streptomycin-resistant Escherichia coli mutant LD1, we confirmed that LD1 is temperature-sensitive for suppression of nonsense codons, and that this phenotype of the mutant and its streptomycin-resistance are genetically linked and are probably caused by a single mutation, strA(LD1). We also isolated a spontaneous revertant, called LD1-R, which partially relieves the restriction of nonsense suppression caused by the strA(LD1) mutation. LD1-R is derived by an additional mutation (revA) which is closely linked to strA(LD1). We further demonstrate that the weak suppression of a lacZUGA mutation in a suppressor-free strain, which probably takes place by normal tRNA1rp, can be detected by the use of the chromagenic substance x-gal (5-Bromo-4-chloro-3-indolyl-beta-D-Galactopyranoside).
Subject(s)
Escherichia coli/genetics , Streptomycin/pharmacology , Suppression, Genetic , Codon , Drug Resistance, Microbial , Genes, Bacterial , Genetic Linkage , Mutation , TemperatureABSTRACT
This paper describes Q beta noninfectious particles produced at 41 degrees C in a streptomycin-resistant Escherichia coli mutant which is temperature sensitive for suppression of a nonsense codon. The noninfectious particles resembled Q beta under the electron microscope and contained coat protein molecules in an amount similar to the amount in Q beta. However, they did not adsorb to F-piliated bacteria, and they were deficient in both minor capsid proteins of Q beta, maturation (IIa) and read-through (IIb). Proteins IIa and IIb were not produced in Qbeta-infected mutant cells at 41 degrees C. In addition, instead of the 30S RNA of Q beta, a shorter RNA, which sedimented mainly at 23 S, was found in the defective particles. The results are discussed in relation to the roles of proteins IIa and IIb of Q beta.
Subject(s)
Coliphages/growth & development , Defective Viruses/growth & development , Escherichia coli/genetics , Mutation , Adsorption , Coliphages/ultrastructure , Defective Viruses/ultrastructure , Drug Resistance, Microbial , Escherichia coli/drug effects , RNA, Viral/analysis , Streptomycin/pharmacology , Temperature , Viral Proteins/analysis , Virus ReplicationABSTRACT
A spontaneous streptomycin-resistant Escherichia coli mutant which is temperature-sensitive for suppression of a nonsense codon was studied for its ability to propagate phages T2, T4D, T5, phi K, f2, MS2, R17, Q beta, lambda as well as filamentous phages fl, fd and M13. Of all phages tested, only the growth of Q beta, lambda, and filamentous phages is inhibited in the mutant at 42 degree C. This selective inhibition suggests that, like Q beta, lambda and filamentous phages also require a read-through proten(s) which results from suppression of a termination codon.
Subject(s)
Coliphages/growth & development , Escherichia coli/genetics , Suppression, Genetic , Drug Resistance, Microbial , Genetic Code , Lysogeny , Mutation , Peptide Chain Termination, Translational , Phenotype , Streptomycin/pharmacologyABSTRACT
A streptomycin-resistant Escherichia coli mutant has been isolated that is temperature sensitive for Qbeta phage, but not for the group I RNA phages f2, MS2, and R17. The growth of Qbeta in the mutant at the nonpermissive temperature (42 degrees C) results in the release of a near-normal burst of noninfectious particles that cosediment with Qbeta in a sucrose gradient. It is assumed that the mutant is defective at elevated temperatures in the suppression of nonsense codons, thereby producing Qbeta-like particles which are noninfectious because of the lack of the read-through protein A1.