Selective expression of a unique glutathione S-transferase Yb3 gene in rat brain.

A cDNA clone from a rat brain lambda gt11 expression vector library contained the entire open reading frame, the 3' untranslated sequence, and 18 nucleotides of the 5' untranslated region of a Yb-type glutathione S-transferase that was clearly distinct from previously characterized liver forms. This form was designated as Yb3. Primer extension analysis indicated that the mRNA for Yb3 extends 118 nucleotides upstream of the translation initiation codon, and its total length of 1308 nucleotides is substantially larger than the transcripts of any of the liver glutathione S-transferases described thus far. The open reading frame of 654 nucleotides of Yb3 is the same as that of liver Yb1 and Yb2, with greater than 80% sequence homology. The amino acid sequence derived for Yb3 had regions conserved in all forms of Yb glutathione S-transferase, some other amino acid residues common to either Yb1 or Yb2, and 28 substitutions unique to Yb3. The 533 nucleotides in the 3' untranslated region of Yb3 were longer and not homologous to the 3' noncoding ends of the previously described glutathione S-transferases. A second clone from the brain library was virtually identical to that of the gene encoding the liver Yb1 form of the enzyme. Northern blot analyses of rat brain poly(A)+ RNA with specific Yb1 and Yb3 probes showed that the Yb3 form hybridized with a 1300-nucleotide mRNA, Yb1 with a separate 1100-nucleotide component, and that the two probes did not cross-hybridize. Moreover, while Yb3 is a minor component in liver, testis, and heart with even lesser amounts in spleen, lung, and kidney, it is a major glutathione S-transferase transcript in rat brain.