Abstract
BackgroundPenicillium chrysogenum is a filamentous fungus that is employed as an industrial producer of β–lactams. The high β–lactam titers of current strains is the result of a classical strain improvement program (CSI) starting with a wild-type like strain more than six decades ago. This involved extensive mutagenesis and strain selection for improved β–lactam titers and growth characteristics. However, the impact of the CSI on the secondary metabolism in general remains unknown.ResultsTo examine the impact of CSI on secondary metabolism, a comparative genomic analysis of β-lactam producing strains was carried out by genome sequencing of three P. chrysogenum strains that are part of a lineage of the CSI, i.e., strains NRRL1951, Wisconsin 54-1255, DS17690, and the derived penicillin biosynthesis cluster free strain DS68530. CSI has resulted in a wide spread of mutations, that statistically did not result in an over- or underrepresentation of specific gene classes. However, in this set of mutations, 8 out of 31 secondary metabolite genes (20 polyketide synthases and 11 non-ribosomal peptide synthetases) were targeted with a corresponding and progressive loss in the production of a range of secondary metabolites unrelated to β–lactam production. Additionally, key Velvet complex proteins (LeaA and VelA) involved in global regulation of secondary metabolism have been repeatedly targeted for mutagenesis during CSI. Using comparative metabolic profiling, the polyketide synthetase gene cluster was identified that is responsible for sorbicillinoid biosynthesis, a group of yellow-colored metabolites that are abundantly produced by early production strains of P. chrysogenum.ConclusionsThe classical industrial strain improvement of P. chrysogenum has had a broad mutagenic impact on metabolism and has resulted in silencing of specific secondary metabolite genes with the concomitant diversion of metabolism towards the production of β–lactams.Electronic supplementary materialThe online version of this article (doi:10.1186/s12864-015-2154-4) contains supplementary material, which is available to authorized users.
Highlights
Penicillium chrysogenum is a filamentous fungus that is employed as an industrial producer of β–lactams
Mutational spread in the P. chrysogenum genome during classical strain improvement To examine the mutational impact of the classical strain improvement (CSI) program on the genome of P. chrysogenum, we sequenced the genomes of four P. chrysogenum strains
Based on a comparison of the respective genomes, 455 Single nucleotide polymorphism (SNP) were detected between the NRRL1951 and Wisconsin 54-1255 strain that accumulated during the first period of CSI
Summary
Penicillium chrysogenum is a filamentous fungus that is employed as an industrial producer of β–lactams. For the industrial production of β–lactams, the filamentous fungus Penicillium chrysogenum has been subjected to an intense classical strain improvement (CSI) program that consisted of repeating rounds of mutagenesis and selection. This program started in 1943 with a natural isolated strain NRRL1951 derived from an infected cantaloupe because of its high levels of β–lactam production under submerged fermentation conditions. To achieve production of increasing levels of β–lactams and to ease the purification of the product, a comprehensive mutagenesis was applied to the NRRL1951 B25 strain [1] This included harsh mutagenic techniques like UV irradiation, X-ray, and nitrogen mustard (methyl-bis(β-chloroethyl) amine) treatment as well as selection of the desired spontaneous variants for production and fermentation characteristics. The genetic studies and genome sequencing resulted in an understanding of the mechanisms underlying the enhanced β-lactam production, which included the amplification of the penicillin biosynthesis gene cluster [8], an enhanced amino acid production and various aspects of cellular development [2]
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