Homologous recombination and allele replacement in transformants of Fusarium fujikuroi

The ascomycete Fusarium fujikuroi could be transformed stably to hygromycin resistance only when the transforming plasmid contained a fragment of DNA from the fungus. The transformation frequencies were roughly independent of the sequence of the particular fungal DNA fragment used, of its size (1.8 or 6 kb), and of whether this DNA was present only once in the fungal genom e or about forty times (the genes for ribosomal RNA). The plasmid was integ rated into the fungal genome by homologous recombination in the eighteen tr ansformants tested; ectopic integration was never observed. The carB gene o f F. fujikuroi was cloned and shown to complement unpigmented mutants defic ient in phytoene dehydrogenase. A mutant carB allele was prepared in vitro and used to transform wild-type protoplasts; the transformants contained a genomic duplication and were heterozygous for cauB; the mutant allele repla ced the original wild-type allele when this was spontaneously lost in the t ransformants. This loss was due to gene conversion in some cases and to rec ombination between repeated sequences in others.