Multiple mRNAs of rat brain α-crystallin B chain result from alternative transcriptional initiation

Two major classes of mRNAs for the α-crystallin B chain (or α(B)crystallin), about 0.9 and 1.2 kilobases in length, are expressed in rat brain. To examine the structures of these mRNAs, we isolated cDNA clones from rat brain and genomic DNA from rat liver. Characterization of these clones as well as Northern blot analysis indicated that the various mRNAs differed in the lengths of their 5' leader sequences. RNase protection assays revealed that the gene for α-crystallin B chain contains multiple start sites. The transcriptional start sites of the long mRNAs are preceded by a putative CAAT box and that of the shorter mRNA by a putative TATA box. The shorter mRNA encodes the α-crystallin B chain protein, whereas the longer mRNA contained three extra small open reading frames upstream of the AUG start codon for the protein. The shorter mRNA is abundant in lens, heart, muscle, and kidney, while the longer mRNAs are constitutively expressed at low levels in a wide variety of tissues. The shorter mRNA was increased by treatment with phorbol 12-myristate 13-acetate in rat C6 glioma cells. Since there is only a single copy of the α-crystallin B chain gene, our results indicate that the two classes of mRNAs are generated by alternative transcriptional initiation from different promoters and their expressions are regulated differentially.