TRANSFECTION ANALYSIS OF EXPRESSION OF MESSENGER-RNA ISOFORMS ENCODING THE NUCLEAR AUTOANTIGEN LA SS-B/

Transcription of the gene encoding for the nuclear autoantigen La resu lted in La mRNA isoforms, A promoter switching combined with an altern ative splicing pathway replaced the exon 1 with the exon 1', The exon 1' contained GC-rich regions and an oligo(U) tail of 23 uridine residu es, Moreover, it encoded for three open reading frames upstream of the La protein reading frame. Despite this unusual structure, when exon 1 ' La mRNAs were expressed in transfected cells, both exon 1 and 1' La mRNAs were translated to La protein, whereas the upstream open reading frames of the exon 1' were not translated. In addition to full-length exon 1' La mRNAs 5'-shortened exon 1' La mRNAs were detected. The exo n 1' 5'-starts varied in dependence on the analyzed tissues. Like the full-length exon 1' La mRNA a 5'-shortened exon 1' construct starting downstream of the oligo(U) tail but upstream of the open reading frame s 2 and 3 was also well translated when transfected in mouse cells, Th us all La mRNA forms represent functional La mRNAs.