Streptomyces clavuligerus pSCL4-cured derivates from the Leon-isolate of the type-strain

The large linear replicon pSCL4 in Streptomyces clavuligerus Leon-isolate of the type-strain has been cured using directed targeting with CRISPR-Cas9. The resulting pSCL4-cured strains have large deletions of the chromosome ends and in two of them we have clear sequence evidence of chromosome circularisation. Together with the impaired morphological development of these strains, the fact that pSCL4 is essential for maintenance of the chromosome in its entirety and linear form, indicate that pSCL4 should be considered a second chromosome rather than a dispensable extrachromosomal plasmid.