An X family DNA polymerase protects mitochondrial DNA in the infective filament of Ustilago maydis

Abstract

Ustilago maydis is a phytopathogenic fungus thatinfects the maize plant to provoke the corn smut disease. A morphological switch from yeast to infective filament is an indispensable prerequisite for enabling the infection to take place. This filament is a cell cycle arrested dikaryotic hypha that arises from the conjugation of two compatible yeasts, grows on the plant surface and eventually develops an specific structure called appresorium, required to penetrate the plant tissue. The formation of the filament in U. maydis is driven by a complex transcriptional programme which is mainly orchestrated by a transcriptional regulator called b-factor. One of the direct targets of the b-factor is PolX, a gene encoding a putative DNA polymerase which is strongly expressed upon filament formation (Brachmann et al., Mol Microb 42, 1047). According to its sequence, this polymerase would belong to the X family of DNA polymerases, implicated in DNA repair processes. We have analysed the ability of polX null mutant dikaryotic hyphae to cope with DNA damaging agents and we have found only slight defects in response to oxidative stress. Nevertheless, we have come across that PolX seems to be required for the ability of the dikaryotic hyphae to resume proliferation once the b-programme is repressed. Interestingly, we have also found that PolX is a mitochondrial protein, most likely involved in the repair of damaged mitochondrial DNA. In this communication we will discuss why the induction of the infective hypha needs the presence of a DNA polymerase to protect the mitochondria.