Cloning of a 1.1‐kb Fragment Including srnB+ Gene in the F Plasmid and Isolation of an srnB Mutant

The srnB+ gene, promoting stable RNA degradation at 42 C in the presence of rifampin, was cloned by using pBR322 as a vector; it was located on a 1.1-kilobase (kb) Eco RI/Bam HI fragment between 1.4 and 2.5 kb of the F plasmid. The region between 93.3 and 4.0 kb of the F plasmid was physically mapped by using restriction endonucleases EcoRI, HindIII, BamHI, PstI, and SmaI, with reference to a standard HindIII site in IS3. An srnB1 mutant was isolated from a chimeric plasmid, pOY54, after treatment of its DNA with hydroxylamine. The srnB1 allele on the F fragment of the mutant plasmid was recessive to the wildtype allele. Thermal elevation of cell cultures to 39 C was high enough to promote RNA degradation in strain YS12 carrying plasmid pOY54.