Mutual correction of faulty PCNA subunits in temperature-sensitive lethal mus209 mutants of Drosophila melanogaster

Proliferating cell nuclear antigen (PCNA) functions in DNA replication as a processivity factor for polymerases delta and epsilon, and in multiple DNA repair processes. We describe two temperature-sensitive lethal alleles ( m us209(Bl) and mus209(2735)) of the Drosophila PCNA gene that, at temperatur es permissive for growth, result in hypersensitivity to DNA-damaging agents , suppression of position-effect variegation, and female sterility in which ovaries are underdeveloped and do not produce eggs. We show by mosaic anal ysis that the sterility of mus209(Bl) is partly due to a failure of germ-li ne cells to proliferate. Strikingly;, mus209(Bl) and mus209(2735) interact to restore partial fertility to heteroallelic females, revealing additional roles for PCNA in ovarian development, meiotic recombination, and embryoge nesis. We further show that, although mus209(Bl) and mus209(2735) homozygot es are each defective in repair of transposase-induced DNA doublestrand bre aks in somatic cells, this defect is substantially reversed in the heteroal lelic mutant genotype. These novel mutations map to adjacent sites on the t hree-dimensional structure of PCNA, which was unexpected in the context of this observed interallelic complementation. These mutations, as well as fou r others we describe, reveal new relationships between the structure and fu nction of PCNA.