EXPRESSION OF THE LAMININ-A CHAIN IS DOWN-REGULATED BY A NONCANONICAL POLYADENYLATION SIGNAL

It is well accepted that 3' untranslated regions (UTR) are an essential part of mRNA. However, little is known in detail about the contribution of different regions of 3' UTR on synthesis, stability and translatability of their mRNA. In addition to the highly conserved hexanucleotide AAUAAA, some consensus sequences for 3'-end processing and polyadenylation have been characterized, but most of this work has been done with viral mRNA or beta-globin mRNA. We have studied the influence of the 3' UTR of the mRNA for the three chains A, B 1, B2 of laminin on the expression of a reporter gene (galK). Laminin is a large glycoprotein of basement membranes and all three polypeptide chains are needed in equal amounts for a functional molecule. The three 3' UTR of the laminin mRNA differ widely with respect to length, number of polyadenylation signals and other consensus sequences. Nevertheless, all three 3' UTR reduce the expression of the reporter gene at least three-fold, when the corresponding cDNA sequences are inserted downstream of the reporter gene instead of the 3' UTR of simian virus 40 early genes. The 3' UTR of laminin-A mRNA contains the non-canonical polyadenylation signal AUUAAA which seems to be responsible for the limiting amounts of laminin-A mRNA and protein compared to those for laminin BI and B2 [Speth and Oberbaumer (1993) Exp. Cell Res. 204, 302-3101. Mutation of the laminin-A polyadenylation signal to the canonical form AAUAAA increases expression by a factor of 2.5.