Complete structure and expression of the rat alpha B-crystallin gene.

alpha B-Crystallin, a member of the small heat shock family of proteins, is synthesized as a component of various developmental programs, in response to stress and in a number of pathological states. We have determined the complete structure of the alpha B-crystallin gene (6,806 bp encompassing 2,299 bp upstream from ATG and 859 bp at the 3' end, past the first polyadenylation signal). Comparison of the rat and the human alpha B-crystallin genes reveals significant conservation of the nucleotide sequences in almost all regions except in intron 2. The 1-kb region immediately upstream of ATG shows about 75% overall homology. A 78-bp sequence in the intron 1 and sequences in the 3' untranslated region show about 95% and 85% sequence identity, respectively. Characterization of the expression of this gene in different tissues in the rat by extensive analyses, utilizing primer extension. RNase protection, and rapid amplification of cDNA ends (RACE) revealed a predominant transcription initiation site 44 bp upstream of ATG. Northern analyses with "coding-only" and upstream "noncoding" probes did not support the thesis that heterogeneity in the alpha B-crystallin mRNAs arises from variations in the sequences immediately upstream of the predominant transcription initiation site. Importantly, the known relative levels of alpha B-crystallin protein in different tissues correlate best with the presence of transcripts starting from this initiation site.