A collection of temperature-sensitive mutants of Streptomyces coelicohr A3(2) was isolated. The majority of the mutants showed an osmotically remedial phenotype. Mutants defective in macromolecular synthesis were identified and characterized further. Four mutants were found in which DNA replication was defective, but which continued to synthesize RNA and protein at the restrictive temperature (39 O C ) . The kinetics of cessation of DNA synthesis allowed a tentative identification of slow (initiation) and fast (elongation) stop dna mutants. "he inhibition of DNA replication in the four mutants was found to be reversible on returning to the permissive temperature (30 "C), but only after a delay of about 2 h. Three other mutants were identified which showed not only cessation of DNA replication at the restrictive temperature, but also defects in other macromolecular synthesis events.

Streptomyces coelicolor A3(2) gives rise to spontaneous chloramphenicol sensitive mutants at a frequency of about 0.3% per spore. These mutants are often genetically unstable and give rise to arginine auxotrophs (Arg-) at frequencies of 1-7% per spore. These Arg- mutants usually lack the enzyme argininosuccinate synthetase (one exception was found that lacked ornithine carbamoyltransferase) and were shown to have deleted the corresponding argG gene by hybridisation analysis using a cloned S. cattleya argG gene. The Arg- strains also showed a variety of different DNA amplification and deletion events in a region homologous to an amplified DNA sequence found in spontaneous Arg- mutants in S. lividans 66.

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