Regulation of AUF1 expression via conserved alternatively spliced elementsin the 3 ' untranslated region

The A+U-rich RNA-binding factor AUF1 exhibits characteristics of a trans-ac ting factor contributing to the rapid turnover of many cellular mRNAs, Stru ctural mapping of the AUF1 gene and its transcribed mRNA has revealed alter native splicing events within the 3' untranslated region (3'-UTR), In K562 erythroleukemia cells, we have identified four alternatively spliced AUF1 3 '-UTR variants, including a population of AUF1 mRNA containing a highly con served 107-nucleotide (nt) 3'-UTR exon (exon 9) and the adjacent downstream intron (intron 9), Functional analyses using luciferase-AUF1 3'-UTR chimer ic transcripts demonstrated that the presence of either a spliceable or an unspliceable intron 9 in the 3'-UTR repressed luciferase expression in cis, indicating that intron 9 sequences may down regulate gene expression by tw o distinct mechanisms. In the case of the unspliceable intron, repression o f luciferase expression likely involved two AUF1-binding sequences, since l uciferase expression was increased by deletion of these sites. However, inc lusion of the spliceable intron in the luciferase 3'-UTR down-regulated exp ression independent of the AUF1-binding sequences. This is likely due to no nsense-mediated mRNA decay (NMD) owing to the generation of exon-exon junct ions more than 50 nt downstream of the luciferase termination codon, AUF1 m RNA splice variants generated by selective excision of intron 9 are thus al so likely to be subject to NMD since intron 9 is always positioned >137 nt downstream of the stop codon, The distribution of alternatively spliced AUF 1 transcripts in K562 cells Is consistent with this model of regulated AUF1 expression.