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

It is well accepted that 3' untranslated regions (UTR) are an essentia l part of mRNA. However, little is known in detail about the contribut ion of different regions of 3' UTR on synthesis, stability and transla tability of their mRNA. In addition to the highly conserved hexanucleo tide AAUAAA, some consensus sequences for 3'-end processing and polyad enylation 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 glycoprot ein of basement membranes and all three polypeptide chains are needed in equal amounts for a functional molecule. The three 3' UTR of the la minin mRNA differ widely with respect to length, number of polyadenyla tion signals and other consensus sequences. Nevertheless, all three 3' UTR reduce the expression of the reporter gene at least three-fold, w hen the corresponding cDNA sequences are inserted downstream of the re porter gene instead of the 3' UTR of simian virus 40 early genes. The 3' UTR of laminin-A mRNA contains the non-canonical polyadenylation si gnal AUUAAA which seems to be responsible for the limiting amounts of laminin-A mRNA and protein compared to those for laminin BI and B2 [Sp eth and Oberbaumer (1993) Exp. Cell Res. 204, 302-3101. Mutation of th e laminin-A polyadenylation signal to the canonical form AAUAAA increa ses expression by a factor of 2.5.