Mitochondrial transporter that imports/exports pyrimidine nucleotides into and from mitochondria. Selectively transports cytosine, guanosine, inosine and uridine (deoxy)nucleoside mono-, di-, and triphosphates by antiport mechanism. Catalyzes uniport at much lower rate. May import (deoxy)nucleoside triphosphates in exchange for intramitochondrial (deoxy)nucleoside mono- and diphosphates, thus providing precursors necessary for de novo synthesis of mitochondrial DNA and RNA while exporting products of their catabolism. Participates in mitochondrial genome maintenance, regulation of mitochondrial membrane potential and mitochondrial respiration