Cloning of the nitrate-nitrite reductase gene cluster of Penicillium chrysogenum and use of the niaD gene as a homologous selection marker.

A new homologous transformation system for the filamentous fungus Penicillium chrysogenum is described. The system is based on complementation of niaD mutants using the nitrate reductase structural gene (niaD) of P. chrysogenum. Spontaneous niaD mutants were identified after selection for chlorate resistance, in growth tests and subsequent complementation with the niaD gene of Aspergillus oryzae. The P. chrysogenum niaD gene was isolated from a genomic library using the Aspergillus nidulans niaD gene as a probe. After subcloning of the hybridizing fragment, the vector obtained, pPC1-1, was capable of transforming a P. chrysogenum niaD mutant at an average of 40 transformants per micrograms of circular DNA. Southern analysis of genomic DNA from a number of transformants showed that pPC1-1 DNA was integrated predominantly at sites other than the niaD locus. Using hybridization analysis it was shown that the niaD gene of P. chrysogenum is clustered with the nitrite reductase gene (niiA). From analysis of the nucleotide sequences of parts of the niaD and niiA genes of P. chrysogenum and comparison of these sequences with nucleotide sequences of the corresponding A. nidulans genes it was deduced that the P. chrysogenum genes are divergently transcribed.