Streptomycin-resistant Escherichia coli mutant temperature sensitive for the production of Qbeta-infective particles.

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.