*4.3. Analysis Using Deletions*

Deletion analysis of 5'UTRs, implying construction of truncated variants of sequences and their subsequent fusion with a reporter gene, makes it possible to identify nucleotide contexts decisive for maintaining the structure of RNA that can be in two particular conformations, as for example, in riboswitches (Figure 7c). Note that an important specific feature of the riboswitches is their ability to both activate and inhibit translation from the controlled ORF, due to the presence of a specific regulatory region, the aptamer. This region with a particular secondary and sometimes tertiary structure, has the properties of a receptor for small molecules (ligands). In the overwhelming majority of cases, riboswitches reside in 5'UTRs. A deletion analysis of the *A. thaliana* phytoene synthase (PSY) 5'UTR has shown that the long 5'UTR variants (403 and 330 nucleotides) with two predicted sequences of convertible RNA conformations, similar to riboswitches, inhibit translation of the reporter gene, in contrast to the short variant (252 nucleotides) of the PSY 5'UTR, lacking such hairpin structure. This allows the short 5'UTR variant to pass the translational control and rapidly elevate the protein levels [54].
