The nuclear autoantigen La/SS-associated antigen B: one gene, three functional mRNAs.

Transcription of the gene encoding for the nuclear autoantigen La resulted in three mRNA forms. A promoter switching combined with an alternative splicing pathway replaced exon 1 with either exon 1´ or exon 1´´. The exon 1´´ donor splice site was located 4 nts downstream of the exon 1´ donor splice site. All three La mRNA forms were expressed in all the tissues analysed including peripheral blood lymphocytes, liver, fetal spleen, cultured primary endothelial cells, and mouse LTA cell lines permanently transfected with the human La gene. Both the exons 1´ and 1´´ had unusual structures. They contained GC-rich regions and an oligo(U)-tail of 23 uridine residues. Moreover, they encoded for three open reading frames upstream of the La protein reading frame. In spite of this unusual structure, when exon 1´ or exon 1´´ La mRNAs were expressed in transfected mouse LTA cells, both La mRNAs were translated to nuclear La protein, indicating that all La mRNA forms are functional mRNAs.