Characterization of the α₄ integrin gene promoter

A cDNA for the α₄ chain of the α₄β₁ integrin was described previously [Takada, Y., Elices, M. J., Crouse, C. & Hemler, M. E. (1989) EMBO J. 8, 1361-1368]. Primer extension analysis indicated that α₄ mRNA extended well beyond the 5′ end of this cDNA. To clone this 5′ sequence, a primer extension cDNA library was constructed, and a cDNA extending an additional 660 base pairs was isolated. This cDNA hybridized to multiple mRNAs in both T and B lymphocytes, but no α₄ mRNA was found in different tissues or in adherent cell lines. A single α₄ gene was detected in a genomic Southern blot when hybridization was done at high stringency; however, additional bands were observed at lower stringency, indicating the presence of α₄-related genes. Some of the different mRNAs that hybridize to the α₄ cDN A may then be the products of these related genes. Analysis of the α₄ genomic sequence revealed a large first exon of 958 base pairs. Interestingly, translation of α₄ initiates from the second ATG in this exon (nucleotide +744). The first ATG (nucleotide +21) is followed by a termination codon 21 amino acids downstream. Such upstream ATG codons have been implicated in translational control of protooncogenes. One major transcriptional start site was identified by using S1 nuclease and primer extension mapping. Consensus sequences for DNA regulatory elements were found upstream of the gene and in exon 1 and intron 1. The α₄ gene 5′ flanking region acted as a promoter in transfection assays. Detailed characterization of the promoter should provide insight into molecular events regulating expression and tissue specificity of α₄.