Sharing of transcription factors after gene duplication in the yeast Saccharomyces cerevisiae

Penicillium chrysogenum, a Vintage Model with a Cutting-Edge Profile in Biotechnology

The discovery of penicillin entailed a decisive breakthrough in medicine. No other medical advance has ever had the same impact in the clinical practise. The fungus Penicillium chrysogenum (reclassified as P. rubens) has been used for industrial production of penicillin ever since the forties of the past century; industrial biotechnology developed hand in hand with it, and currently P. chrysogenum is a thoroughly studied model for secondary metabolite production and regulation. In addition to its role as penicillin producer, recent synthetic biology advances have put P. chrysogenum on the path to become a cell factory for the production of metabolites with biotechnological interest. In this review, we tell the history of P. chrysogenum, from the discovery of penicillin and the first isolation of strains with high production capacity to the most recent research advances with the fungus. We will describe how classical strain improvement programs achieved the goal of increasing production and how the development of different molecular tools allowed further improvements. The discovery of the penicillin gene cluster, the origin of the penicillin genes, the regulation of penicillin production, and a compilation of other P. chrysogenum secondary metabolites will also be covered and updated in this work.

Aspects on evolution of fungal beta-lactam biosynthesis gene clusters and recruitment of trans-acting factors

Penicillins and cephalosporins are beta-lactam antibiotics. The formation of hydrophobic penicillins has been reported in fungi only, notably Penicillium chrysogenum and Aspergillus (Emericella) nidulans, whereas the hydrophilic cephalosporins are produced by both fungi, e.g., Acremonium chrysogenum (cephalosporin C), and bacteria. The producing bacteria include Gram-negatives and Gram-positives, e.g., Streptomyces clavuligerus (cephamycin C) and Lysobacter lactamgenus (cephabacins), respectively. The evolutionary origin of beta-lactam biosynthesis genes has been the subject of discussion for many years, and two main hypotheses have been proposed: (i) horizontal gene transfer (HGT) from bacteria to fungi or (ii) vertical decent. There are strong arguments in favour of HGT, e.g., unlike most other fungal genes, beta-lactam biosynthesis genes are clustered and some of these genes lack introns. In contrast to S. clavuligerus, all regulators of fungal beta-lactam biosynthesis genes represent wide-domain regulators that are not part of the gene cluster. If bacterial regulators were co-transferred with the gene cluster from bacteria to fungi, most likely they would have been non-functional in eukaryotes and lost during evolution. Recently, the penicillin biosynthesis gene aatB was discovered, which is not part of the penicillin biosynthesis gene cluster and is even located on a different chromosome. The aatB gene is regulated by the same regulators AnCF and AnBH1 as the penicillin biosynthesis gene aatA (penDE). Data suggest that aatA and aatB are paralogues derived by duplication of a common ancestor gene. This data supports a model in which part of the beta-lactam biosynthesis gene cluster was transferred to some fungi, i.e., the acvA and ipnA gene without a regulatory gene. We propose that during the assembly of aatA and acvA-ipnA into a single gene cluster, recruitment of transcriptional regulators occurred along with acquisition of the duplicated aatA ancestor gene and its cis-acting sites.

Identification of the novel penicillin biosynthesis gene aatB of Aspergillus nidulans and its putative evolutionary relationship to this fungal secondary metabolism gene cluster

The final step of penicillin biosynthesis in the filamentous fungus Aspergillus nidulans is catalysed by isopenicillin N acyltransferase encoded by the aatA gene. Because there is no bacterial homologue, its evolutionary origin remained obscure. As shown here,disruption of aatA still enabled penicillin production. Genome mining led to the discovery of the aatB gene(AN6775.3) which has a similar structure and expression pattern as aatA. Disruption of aatB resulted in a reduced penicillin titre. Surface plasmon resonance analysis and Northern blot analysis indicated that the promoters of both aatA and aatB are bound and regulated by the same transcription factors AnCF and AnBH1f. In contrast to aatA, aatB does not encode a peroxisomal targeting signal (PTS1). Overexpression of a mutated aatB(PTS1) gene in an aatA-disruption strain(leading to peroxisomal localization of AatB)increased the penicillin titre more than overexpression of the wild-type aatB. Homologues of aatA are exclusively part of the penicillin biosynthesis gene cluster,whereas aatB homologues also exist in non-producing fungi. Our findings suggest that aatB is a paralogue of aatA. They extend the model of evolution of the penicillin biosynthesis gene cluster by recruitment of a biosynthesis gene and its cis-regulatory sites upon gene duplication.

Pervasive and persistent redundancy among duplicated genes in yeast

The loss of functional redundancy is the key process in the evolution of duplicated genes. Here we systematically assess the extent of functional redundancy among a large set of duplicated genes in Saccharomyces cerevisiae. We quantify growth rate in rich medium for a large number of S. cerevisiae strains that carry single and double deletions of duplicated and singleton genes. We demonstrate that duplicated genes can maintain substantial redundancy for extensive periods of time following duplication (∼100 million years). We find high levels of redundancy among genes duplicated both via the whole genome duplication and via smaller scale duplications. Further, we see no evidence that two duplicated genes together contribute to fitness in rich medium substantially beyond that of their ancestral progenitor gene. We argue that duplicate genes do not often evolve to behave like singleton genes even after very long periods of time.

Gene duplication is the primary source of new genes. To persist, duplicated genes must lose some of the original redundancy either by partitioning the ancestral function (subfunctionalization) or by gaining new non-redundant functions (neofunctionalization). The extent to which these processes shape the evolution of duplicated genes over long periods of time is unknown. We investigate these questions experimentally by building strains carrying single and double gene deletions of duplicated genes and measuring their growth rates in rich medium. Using these data, we determine that many duplicated genes are functionally redundant to a substantial degree. We also investigate how often duplicated genes gain new functionality. We demonstrate that the fitness effects of double deletions of duplicate genes are indistinguishable from our best estimate of the fitness effects of deletions of their ancestral singleton genes. We therefore argue that many duplicate genes do not gain substantial new functionality at least in the rich medium. Our results suggest that subfunctionalization does not generally proceed to completion, even after very long periods of time, and that neofunctionalization is either rare or of little consequence, at least under some growth conditions.