Hu proteins have been shown to bind to AU-rich elements (AREs) in the 
3'-untranslated region of unstable mRNAs. They can thereby inhibit the decay of 
labile transcripts by antagonizing destabilizing proteins that target these 
AU-rich sequences. Here we examine the sequence preferences of HuD to elucidate 
its possible role in counteracting mRNA decay. Using repeats of the prototype 
destabilizing sequence UU(AUUU)nAUU, we show that all three HuD RNA-binding 
domains participate in binding to AU-tracts that can be as short as 13 residues, 
depending on the position of the remaining As. Removal of the A residues, 
resulting in a poly(U)-tract, increased the affinity of HuD for RNA, suggesting 
that the presence of As in destabilizing elements might favor the recruitment of 
other proteins and/or prevent HuD from binding too tightly to AREs. In vitro 
selection experiments with randomized RNAs confirmed the preference of HuD for 
poly(U). RNA binding analysis of the related protein HuB showed a similar 
preference for poly(U). In contrast, tristetraprolin, an mRNA destabilizing 
protein, strongly prefers AU-rich RNA. Many labile mRNAs contain U-tracts in or 
near their AREs. Individual AREs may thus differentially affect mRNA half-life 
by recruiting a unique complement of stabilizing and destabilizing factors.