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Background  Multi-copy gene integration into microbial genomes is a conventional tool for obtaining improved gene expression. For  Penicillium chrysogenum , the fungal producer of the beta-lactam antibiotic penicillin, many production strains carry multiple copies of the penicillin biosynthesis gene cluster. This discovery led to the generally accepted view that high penicillin titers are the result of multiple copies of penicillin genes. Here we investigated strain P2niaD18, a production line that carries only two copies of the penicillin gene cluster.    Results  We performed pulsed-field gel electrophoresis (PFGE), quantitative qRT-PCR, and penicillin bioassays to investigate production, deletion and overexpression strains generated in the  P. chrysogenum  P2niaD18 background, in order to determine the copy number of the penicillin biosynthesis gene cluster, and study the expression of one penicillin biosynthesis gene, and the penicillin titer. Analysis of production and recombinant strain showed that the enhanced penicillin titer did not depend on the copy number of the penicillin gene cluster. Our assumption was strengthened by results with a penicillin null strain lacking  pcbC  encoding isopenicillin N synthase. Reintroduction of one or two copies of the cluster into the  pcbC  deletion strain restored transcriptional high expression of the  pcbC  gene, but recombinant strains showed no significantly different penicillin titer compared to parental strains.    Conclusions  Here we present a molecular genetic analysis of production and recombinant strains in the P2niaD18 background carrying different copy numbers of the penicillin biosynthesis gene cluster. Our analysis shows that the enhanced penicillin titer does not strictly depend on the copy number of the cluster. Based on these overall findings, we hypothesize that instead, complex regulatory mechanisms are prominently implicated in increased penicillin biosynthesis in production strains.    Electronic supplementary material  The online version of this article (doi:10.1186/s12896-017-0335-8) contains supplementary material, which is available to authorized users.

Penicillin production in industrial strain Penicillium chrysogenum P2niaD18 is not dependent on the copy number of biosynthesis genes