Pathways of transformation in Ustilago maydis determined by DNA conformation.

Ustilago maydis was transformed by plasmids bearing a cloned, selectable gene but lacking an autonomously replicating sequence. Transformation was primarily through integration at nonhomologous loci when the plasmid DNA was circular. When the DNA was made linear by cleavage within the cloned gene, the spectrum of integration events shifted from random to targeted recombination at the resident chromosomal allele. In a large fraction of the transformants obtained using linear DNA, the plasmid DNA was not integrated but was maintained in an extrachromosomal state composed of a concatameric array of plasmid units joined end-to-end. The results suggest the operation of several pathways for transformation in U. maydis, and that DNA conformation at the time of transformation governs choice of pathways.