Complex sentence: Rebeccamycin is a weak topoisomerase I inhibitor isolated from "Streptomyces sp." It is structurally similar to staurosporine, but does not show any inhibitory activity against protein kinases.
Split into simpler sentences: Rebeccamycin is a weak topoisomerase I inhibitor isolated from "Streptomyces sp." structurally similar to staurosporine. Does not show any inhibitory activity against protein kinases.