Construction of a novel fungal gus expression plasmid, and its evaluation in Aspergillus nidulans : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Genetics at Massey University

Abstract

A GUS expression plasmid, pFunGus, was constructed containing a multi-cloning site for the insertion of gene regulatory elements, to be used in fungal reporter gene studies. A derivative of pFunGus (pFG-gpd) was constructed by the insertion of the gpdA promoter (glyceradehyde-3-phosphatc dehydrogenase) into the multi-cloning site of pFunGus for the assessment of the plasmid's transformation and expression properties in Aspergillus niduans. The correct construction of pFunGus and pFG-gpd was verified by analytical restriction digests and by its property of GUS expression in A. nidulans. The plasmid was integrated into the A. nidulans genome via cotransformation with the phleomycin resistance plasmid, pAN8-l. Transformation frequencies of between 3 and 250 transformants per µg of pAN8-l DNA were obtained. Initial screening for cotransformation yielded no pFG-gpd transformants. Attempts to improve cotransformation frequencies by optimisation of cotransformation conditions were unsuccessful. However, large scale screenings of transformants lead to cotransformants being isolated at a very low cotransformation frequency. Approximately 0.45% of pAN8-l transformants possessed the GUS phenotype. The eight pFG-gpd transformants obtained were analysed by Southern hybridisation. Six out of the eight transformants had a single copy integration. Of the remaining two transformants, one had three copies integrated at separate locations, one of which was disrupted, and the other had four copies integrated as tandem repeats, one of which was disrupted. All the transforming DNA appeared to be integrated ectopically. The physiology of the transformants was assessed by dry weight increase, colony extension and total protein content. These showed that the transformants biology was not significantly compromised by the transforming DNA. Finally, high levels of GUS expression were observed in all pFG-gpd transformants and the GUS expression per copy of the GUS expression cassette integrated into the genome was constant. These results showed that the transformed gene copy number determined the levels of gene activity rather than the position of integration in the genome. Overall these results demonstrate the potential application of the versatile GUS expression plasmid, pFunGus for reporter gene studies in filamentous fungi.