The untranslated exon B of human surfactant protein A2 mRNAs is an enhancer for transcription and translation

Two human genes, SFTPA1 (SP-A1) and SFTPA2 (SP-A2), encode surfactant protein A, a molecule of innate immunity and surfactant-related functions. Several genetic variants have been identified for both genes. These include nucleotide (nt) polymorphisms, as well as alternative splicing patterns at the 5' untranslated region (5'UTR). Exon B (eB) is included in the 5'UTR of most SP-A2, but not SP-A1 splice variants. We investigated the role of eB in the regulation of gene expression and translation efficiency. A luciferase (Luc) reporter gene was cloned downstream of the entire (AeBD) or eB deletion mutants (del_mut) of the SP-A2 5'UTR, or heterologous 5'UTRs containing the eB sequence, or a random sequence of equal length. The del_mut constructs consisted in consecutive deletions of five nucleotides (n = 8) within eB and the exon-exon junctions in the AeBD 5'UTR. Luc activities and mRNA levels were compared after transfection of NCI-H441 cells. We found that 1) eB increased Luc mRNA levels when placed upstream of heterologous 5'UTR sequences or the promoter region, regardless of its position and orientation; 2) translation efficiency of in vitro-generated mRNAs containing eB was higher than that of mRNAs without eB; and 3) the integrity of eB sequence is crucial for transcription and translation of the reporter gene. Thus eB 1) is a transcription enhancer, because it increases mRNA content regardless of position and orientation, 2) enhances translation when placed in either orientation within its natural 5'UTR sequence and in heterologous 5'UTRs, and 3) contains potential regulatory elements for both transcription and translation. We conclude that eB sequence and length are determinants of transcription and translation efficiency.