The MALAT1 (metastasis-associated lung adenocarcinoma transcript 1) locus is 
misregulated in many human cancers and produces an abundant long 
nuclear-retained noncoding RNA. Despite being transcribed by RNA polymerase II, 
the 3' end of MALAT1 is produced not by canonical cleavage/polyadenylation but 
instead by recognition and cleavage of a tRNA-like structure by RNase P. Mature 
MALAT1 thus lacks a poly(A) tail yet is expressed at a level higher than many 
protein-coding genes in vivo. Here we show that the 3' ends of MALAT1 and the 
MEN β long noncoding RNAs are protected from 3'-5' exonucleases by highly 
conserved triple helical structures. Surprisingly, when these structures are 
placed downstream from an ORF, the transcript is efficiently translated in vivo 
despite the lack of a poly(A) tail. The triple helix therefore also functions as 
a translational enhancer, and mutations in this region separate this translation 
activity from simple effects on RNA stability or transport. We further found 
that a transcript ending in a triple helix is efficiently repressed by microRNAs 
in vivo, arguing against a major role for the poly(A) tail in microRNA-mediated 
silencing. These results provide new insights into how transcripts that lack 
poly(A) tails are stabilized and regulated and suggest that RNA triple-helical 
structures likely have key regulatory functions in vivo.