The fi-carboline alkaloids are a large group of natural and synthetic indole alkaloids with various pharmacological properties. Previously, the gene cluster of streptonigrin biosynthesis (stn) was identified in Streptomyces flocculus CGMCC4.1223, revealing a hypothetical protein (StnK4). In this work, a mutant dstnK4 was constructed by genetic manipulation and the mutant produced three fi-carboline alkaloids, including two known compounds (oxoproopaline D and oxoproopaline G) and one new compound, namely oxoproopaline H. Structure of the new fi-carboline was established by spectroscopic analyses. A biosynthetic pathway for these fi-carboline alkaloids was proposed. (c) 2022 Elsevier Ltd. All rights reserved.