Dominant Suppression of Repeat-Induced Point Mutation in Neurospora crassa by a Variant Catalytic Subunit of DNA Polymerase-ζ

Abstract

Abstract Crosses involving the Adiopodoumé strain of Neurospora crassa are defective for repeat-induced point mutation (RIP), a genome defense mechanism of fungi. We show here that the Adiopodoumé strain possesses an incompletely penetrant and variably expressive dominant suppressor of RIP (Srp) that maps to an ∼34-kbp genome segment that is ∼26 kbp proximal to mat on linkage group IL. Gene disruption experiments revealed that Srp is the upr-1 allele of Adiopodoumé (upr-1Ad) that is contained within this segment. The upr-1 gene codes for the catalytic subunit of the translesion DNA polymerase-ζ (Pol-ζ) and it is unusually polymorphic in Neurospora. That the upr-1 gene contains upstream ORFs that overlap with the main ORF is potentially relevant to the incomplete penetrance and variable expressivity of the suppressor. Crosses between heterokaryons that contain upr-1Ad and strains that prevent mating events involving nuclei that contain upr-1Ad yielded no progeny in which RIP had occurred, consistent with the idea that the suppressor encoded by upr-1Ad is diffusible. The potential involvement of the Pol-ζ subunit in two functions, translesion DNA synthesis and RIP regulation, might account for the rapid evolution of its gene in Neurospora.