Heavily methylated amplified DNA in transformants of Neurospora crassa

Abstract

Substantial DNA methylation occurs in higher eukaryotes and in some cases affects gene expression. However, the genomes of some fungi, Drosophila and other lower eukaryotes have an extremely low 5-methylcytosine content, suggesting that DNA methylation might not have a general role in gene control. We have now found heavy methylation of transforming DNA which has become stably amplified in complex tandem arrays in the fungus Neurospora crassa. Rearranged amplified arrays of this type have not previously been found in fungi, but resemble those of animal system. Our results demonstrate that this lower eukaryote normally maintains only very low levels of 5-methylcytosine in its genome, but possesses a mechanism for substantial methylation of DNA de novo. This heavy methylation, which lacks the preference for CG sequences found in higher eukaryotes, does not apparently affect gene expression and might be involved in a recombination or repair process for which the amplified DNA is a target.