Isolation and complementation of mutants of Streptomyces coelicolor “Müller” DSM3030 deficient in lysozyme production

Streptomyces coelicolor “Muller” is known to excrete the lysozyme N-acetylmuramidase. Culture filtrates of this strain form a characteristic halo on agar plates containing freeze-dried Micrococcus luteus cells (lysoplate technique). The halo consists of a clear inner zone and a turbid outer ring. Simulation experiments showed that the turbid outer ring is most probably produced by lysozyme whereas the clear inner zone can be considered to be due to an additional protease action. Using the lysoplate technique UV- and NTG-mutagenized strains of S. coelicolor “Muller” were screened for mutants defective in lysozyme production. Two mutants, SC11 and SC12, were identified. The mutant SC11 was selected for complementation studies. First, a transformation system was established. The use of a soft-agar overlay method was necessary to yield high regeneration rates of SC11 protoplasts. The plasmid vector pEB15 could be transferred into this mutant strain with an efficiency of 105 transformants/μg DNA. The high efficiency allowed shot-gun cloning experiments. Genomic DNA of S. coelicolor “Muller” digested with Sau3A and inserted into pEB15 was introduced into the mutant SC11. A complemented mutant was identified. A 2.9 kilobase pair (kb) DNA fragment was found which restored the lysozyme production of both mutants, SC11 and SC12. According to the diameter of the produced halos the complemented mutant SC11 was suggested to produce more lysozyme than the wildtype strain.