Identification of the genes encoding the catalytic steps corresponding to LRA4 (l-2-keto-3-deoxyrhamnonate aldolase) and l-lactaldehyde dehydrogenase in Aspergillus nidulans: evidence for involvement of the loci AN9425/lraD and AN0544/aldA in the l-rhamnose catabolic pathway

The Loss-of-Function Mutation aldA67 Leads to Enhanced α-L-Rhamnosidase Production by Aspergillus nidulans

In Aspergillus nidulans L-rhamnose is catabolised to pyruvate and L-lactaldehyde, and the latter ultimately to L-lactate, via the non-phosphorylated pathway (LRA) encoded by the genes lraA-D, and aldA that encodes a broad substrate range aldehyde dehydrogenase (ALDH) that also functions in ethanol utilisation. LRA pathway expression requires both the pathway-specific transcriptional activator RhaR (rhaR is expressed constitutively) and the presence of L-rhamnose. The deletion of lraA severely impairs growth when L-rhamnose is the sole source of carbon and in addition it abolishes the induction of genes that respond to L-rhamnose/RhaR, indicating that an intermediate of the LRA pathway is the physiological inducer likely required to activate RhaR. The loss-of-function mutation aldA67 also has a severe negative impact on growth on L-rhamnose but, in contrast to the deletion of lraA, the expression levels of L-rhamnose/RhaR-responsive genes under inducing conditions are substantially up-regulated and the production of α-L-rhamnosidase activity is greatly increased compared to the aldA⁺ control. These findings are consistent with accumulation of the physiological inducer as a consequence of the loss of ALDH activity. Our observations suggest that aldA loss-of-function mutants could be biotechnologically relevant candidates for the over-production of α-L-rhamnosidase activity or the expression of heterologous genes driven by RhaR-responsive promoters.

Agrobacterium tumefaciens-Mediated Transformation of NHEJ Mutant Aspergillus nidulans Conidia: An Efficient Tool for Targeted Gene Recombination Using Selectable Nutritional Markers

Protoplast transformation for the introduction of recombinant DNA into Aspergillus nidulans is technically demanding and dependant on the availability and batch variability of commercial enzyme preparations. Given the success of Agrobacterium tumefaciens-mediated transformation (ATMT) in diverse pathogenic fungi, we have adapted this method to facilitate transformation of A. nidulans. Using suitably engineered binary vectors, gene-targeted ATMT of A. nidulans non-homologous end-joining (NHEJ) mutant conidia has been carried out for the first time by complementation of a nutritional requirement (uridine/uracil auxotrophy). Site-specific integration in the ΔnkuA host genome occurred at high efficiency. Unlike other transformation techniques, however, cross-feeding of certain nutritional requirements from the bacterium to the fungus was found to occur, thus limiting the choice of auxotrophies available for ATMT. In complementation tests and also for comparative purposes, integration of recombinant cassettes at a specific locus could provide a means to reduce the influence of position effects (chromatin structure) on transgene expression. In this regard, targeted disruption of the wA locus permitted visual identification of transformants carrying site-specific integration events by conidial colour (white), even when auxotrophy selection was compromised due to cross-feeding. The protocol described offers an attractive alternative to the protoplast procedure for obtaining locus-targeted A. nidulans transformants.