The deafness-associated A7445G mutation in the precursor of mitochondrial tRNA(Ser(UCN)) has been identified in several pedigrees from different ethnic backgrounds. To determine the role of nuclear background in the biochemical manifestation associated with the A7445G mutation, we performed a biochemical characterization of this mutation using cybrids constructed by transferring mitochondria from lymphoblastoid cell lines derived from a New Zealand family into human osteosarcoma mtDNA-less (p(0)) cells. Compared with three control cybrids, three cybrids derived from an affected matrilineal relative carrying the homoplasmic A7445G (Ser(UCN)) mutation exhibited similar to38-57% decrease in the steady-state level of tRNA, which is less reduced levels than in lymphoblastoid cells in the previous study. Furthermore, similar to22% reduction in the level of aminoacylation of tRNA (Ser(UCN)) was observed in the mutant cybrid cells. Interestingly, similar to60-63% decrease of steady-state level of ND6 gene, which belongs to the same precursor as that of tRNA(Ser(UCN)), in cybrid cell lines carrying the A7445G mutation, is more than that observed in lymphoblastoid cells. These observations strongly point out a mechanistic link between the processing defect of the tRNA(Ser(UCN)) precursor and decreased stability of ND6 mRNA precursor. These results also imply the influence of nuclear background on the biochemical phenotype associated with the A7445G mutation. (C) 2005 Elsevier Inc. All rights reserved.