Novel structural determinants in human SECIS elements modulate the translational recoding of UGA as selenocysteine

The selenocysteine insertion sequence (SECIS) element directs the translational recoding of UGA as selenocysteine. In eukaryotes, the SECIS is located downstream of the UGA codon in the 3′-UTR of the selenoprotein mRNA. Despite poor sequence conservation, all SECIS elements form a similar stem-loop structure containing a putative kink-turn motif. We functionally characterized the 26 SECIS elements encoded in the human genome. Surprisingly, the SECIS elements displayed a wide range of UGA recoding activities, spanning several 1000-fold in vivo and several 100-fold in vitro. The difference in activity between a representative strong and weak SECIS element was not explained by differential binding affinity of SECIS binding Protein 2, a limiting factor for selenocysteine incorporation. Using chimeric SECIS molecules, we identified the internal loop and helix 2, which flank the kink-turn motif, as critical determinants of UGA recoding activity. The simultaneous presence of a GC base pair in helix 2 and a U in the 5′-side of the internal loop was a statistically significant predictor of weak recoding activity. Thus, the SECIS contains intrinsic information that modulates selenocysteine incorporation efficiency.     

Big eucalypts grow more slowly in a warm climate: evidence of an interaction between tree size and temperature

Large trees are critical components of forest ecosystems, but are declining in many forests worldwide. We predicted that growth of large trees is more vulnerable than that of small trees to high temperatures, because respiration and tissue maintenance costs increase with temperature more rapidly than does photosynthesis and these costs may be disproportionately greater in large trees. Using 5 00 000 measurements of eucalypt growth across temperate Australia, we found that high temperatures do appear to impose a larger growth penalty on large trees than on small ones. Average stem diameter growth rates at 21 °C compared with 11 °C mean annual temperature were 57% lower for large trees (58 cm stem diameter), but only 29% lower for small trees (18 cm diameter). While our results are consistent with an impaired carbon budget for large trees at warmer sites, we cannot discount causes such as hydraulic stress. We conclude that slower growth rates will impede recovery from extreme events, exacerbating the effects of higher temperatures, increased drought stress and more frequent fire on the tall eucalypt forests of southern Australia.