CDNA CLONING OF RAT MAJOR AP ENDONUCLEASE (APEX NUCLEASE) AND ANALYSES OF ITS MESSENGER-RNA EXPRESSION IN RAT-TISSUES

APEX nuclease is a mammalian DNA repair enzyme having apurinic/apyrimi dinic (AP) endonuclease, 3'-5'-exonuclease, DNA 3' repair diesterase a nd DNA 3'-phosphatase activities. It is also a redox factor (Ref-1), s timulating DNA binding activity of AP-1 binding proteins such as Fos a nd Jun. In the present paper, a cDNA for the enzyme was isolated from a rat brain cDNA library using mouse Apex cDNA as a probe and sequence d. The rat Apex cDNA was 1221 nucleotides (nt) long, with a 951-nt cod ing region. The amino acid sequence of rat APEX nuclease has 98.4% ide ntity with mouse APEX nuclease. Using the rat Apex cDNA as a probe for Northern blot analysis, the size of rat Apex mRNA was shown to be app roximately 1.5 kb. Its expression was compared in 9 rat organs on post natal days 7 and 28. Although Apex mRNA was expressed ubiquitously, th e levels varied significantly, suggesting organ- or tissue-specific ex pression of the Apex gene. The highest level was observed in the testi s, relatively high levels in the thymus, spleen, kidney and brain, and the lowest level in the liver. The level of expression at postnatal d ay 28, with the exception of the testis, was almost the same as or low er in respective organs than that at postnatal day 7. Postnatal develo pmental changes of Apex mRNA expression in the testis and thymus were further studied. The expression in testis was markedly increased on po stnatal days 21 and 28. The expression in thymus increased once at pos tnatal day 14, and then decreased. The developmental changes of Apex m RNA expression in testis and thymus suggest that APEX nuclease is invo lved in processes such as recombinational events.