Regulation of erbA(alpha) alternative pre-mRNA processing
Abstract
The erbAα gene codes for two functionally antagonistic nuclear hormone receptors. Alternative processing at the 3' end of the erbAα pre-mRNA produces either α1 mRNA, which codes for the α-thyroid hormone receptor, or α2 mRNA, which codes for a nuclear receptor that blocks thyroid hormone activity. Thus, regulation of erbAα alternative processing is a way for the cell to modulate thyroid hormone responsiveness, a process critical for normal development and growth. Despite the importance for control of α1 and α2 mRNA production, little is known about the regulation of erbAα alternative processing. An unusual feature of the erbAα gene is the presence of another gene, RevErb, encoded on the opposite DNA strand. RevErb also codes for a physiologically relevant nuclear hormone receptor. Importantly, RevErb overlaps with α2 sequence but not with α1. As a result, RevErb and α2 may basepair to form an antisense:sense RNA duplex that negatively affects α2 expression. An antisense mechanism for regulation of gene expression has not been well characterized in eukaryotes. However, the discovery of numerous naturally-occurring complementary RNAs and the effective use of artificial antisense RNAs to manipulate gene expression suggest that antisense RNA is a mechanism for control of eukaryotic gene expression. The control of basal erbAα alternative pre-mRNA processing as well as regulation of α2 mRNA by RevErb expression was examined. I found that a balance between α1 and α2 mRNA processing requires multiple cis -acting sequence elements. A non-consensus α2 5'ss allows the α1 polyadenylation signal to compete for processing. Additionally, a splicing enhancer sequence, SEα2, is required for recognition of the sub-optimal α2 5'ss. Several lines of evidence indicate that RevErb affects erbAα processing. The differential expression of RevErb during B cell maturation and the correlation between high RevErb and low α2 mRNA levels supports the hypothesis that RevErb is involved in the developmental and tissue specific regulation of erbAα expression. Furthermore, erbAα/RevErb minigene expression in cell culture shows that RevErb causes a two-fold decrease in α2 from erbAα pre-mRNA. Preliminary results also identify modifications within α2 RNA that suggest RevErb and α2 form antisense/sense RNA duplexes in vivo .
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