Structural features within the NORAD long noncoding RNA underlie efficient repression of Pumilio activity

It is increasingly appreciated that long non-coding RNAs (lncRNAs) carry out important functions in mammalian cells, but how these are encoded in their sequences, and manifested in their structures remains largely unknown. Some lncRNAs bind to and modulate the availability of RNA binding proteins, but the structural principles that underlie this mode of regulation are underexplored. Here, we focused on the NORAD lncRNA, which binds Pumilio proteins and modulates their ability to repress hundreds of mRNA targets. We probed the RNA structure and long-range RNA-RNA interactions formed by NORAD inside cells, under different stressful conditions. We discovered that NORAD structure is highly modular, and consists of well-defined domains that contribute independently to NORAD function. The structure adopted by NORAD spatially clusters the Pumilio binding sites along NORAD in a manner that contributes to de-repression of Pumilio target proteins. Following arsenite stress, the majority of NORAD structure undergoes relaxation and forms inter-molecular interactions with RNAs that are targeted to stress granules. NORAD sequence thus dictates elaborated structural domain organisation that facilitates its function on multiple levels, and which helps explain the extensive evolutionary sequence conservation of NORAD regions that are not predicted to directly bind Pumilio proteins. Examination of NORAD RNA structure in: untreated cells (3 biological replicates and 3 paired controls); cells under DNA damage stress (2 biological replicates and 2 paired controls; and cells under Arsenite stress (3 biological replicates and 3 paired controls) DMS-seq for a region comprising NRU7-8 in NORAD