The expression of the major protein kinase C substrate MARCKS (myristoylated 
alanine-rich C kinase substrate) is controlled by the stability of its mRNA. 
While the MARCKS mRNA is long living in quiescent fibroblasts (t1/2 = 14 h), its 
half-life time is drastically reduced (t1/2 = 2 h) in cells treated with phorbol 
esters to activate protein kinase C (PKC) or treated with growth factors. In a 
first step to study the underlying mechanism we identified both a cis-element on 
the MARCKS mRNA and the corresponding trans-acting factors. Fusing the complete 
3'-UTR or specific regions of the 3'-UTR of the MARCKS gene to a luciferase 
reporter gene caused a drastic decrease in luciferase expression to as low as 
5-10% of controls. This down-regulation was a result of destabilization of the 
chimeric transcript as shown by RNA run-off and Northern blot-assays. By 
RNase/EMSA and UV-cross-linking experiments, we identified a stretch of 52 
nucleotides [(CUUU)11(U)8] in the 3'-UTR of the MARCKS mRNA specifically 
recognized by two RNA-binding proteins, HuD and HuR. These trans-acting factors 
are members of the ELAV gene family and bind the MARCKS CU-rich sequence with 
high affinity. Overexpression of HuD and HuR in murine fibroblasts caused a 
striking stabilization of the endogenous MARCKS mRNA even under conditions when 
the MARCKS mRNA is normally actively degraded, i.e. after treating cells with 
phorbol ester. These data imply, that the identified CU-rich cis-element of the 
MARCKS 3'-UTR is involved in conferring instability to mRNAs and that members of 
the ELAV gene family oppose this effect. Based on its structural and functional 
properties, the (CUUU)11(U)8 sequence described here can be grouped into class 
III of AU-rich elements.