## 2 Addition of Superior Functions to Natural Nucleic Acid through Covalent Conjugations  
Nucleic acids show complementary recognition by base pairing, which cannot be achieved by other biomolecules. A specific region on messenger RNA (mRNA) can be targeted by rationally designing oligonucleotides, such as antisense and small interfering RNA (siRNA), that enable the suppression of the protein expression from the target mRNA. An artificial nucleic acid mimic typified by peptide nucleic acid (PNA) further enables targeting a specific region on genomic double-stranded DNA through the formation of a double-duplex invasion complex. When these oligonucleotides are applied for gene regulation, the delivery of the oligonucleotides into cells and localization in appropriate cellular compartments are issues that need to be addressed. The conjugation of a functional unit for providing cell permeability to oligonucleotides is a promising strategy for the intracellular delivery. [PERSON] et al. synthesized a novel type of lipid-oligonucleotide conjugates (LONs) based on their original chemistry, performing the conjugation on solid support. They demonstrated that siRNAs built of LONs downregulated gene expression through self-intermalization into cells [1]. Since the constructed LONs spontaneously formed particles, the additional functionality of resistance to degradation for in vivo application is expected. Localization in a specific cellular compartment can be achieved by the conjugation of a short peptide tag, such as nuclear localization signal (NLS). [PERSON] and [PERSON] et al. have constructed an NLS-PNA conjugate, which was superior to natural PNA in its invasion efficiency into the target DNA duplex [2]. The demonstration motivates the applicability for in cell studies because the NLS-PNA could form the double-duplex invasion complex even in the presence of physiological salt concentrations, whereas the invasion efficiency of PNA decreased as the salt concentration increased. In addition to the complementary sequence, nucleic acids form complex tertiary structures that recognize other biomolecules such as proteins and small molecule metabolites. Aptamers, nucleic acids with potential to specifically recognize another molecule, have been applied as functional units in biosensors. Biosensors require a unit for recognizing the target molecule and another unit for outputting signal, which can be easily recorded and quantified such as fluorescence and luminescence. Since aptamers only recognize the target molecule, the construction of aptamer-based biosensors has been demonstrated by conjugations with other functional units. For example, the conjugation of an aptamer with a protein is one of the useful strategies because some proteins exhibit unique photochemical output signals. [PERSON] et al. have demonstrated the construction of nucleic acid-protein hybrid molecules using a protein involved in viral replication initiation that acts as a conjugation unit. They constructed an aptamer-protein hybrid that detects the target molecule by a bioluminescence resonance energy transfer (BRET) through designing a fusion protein of the conjugation unit between NanoLuc luciferase and Venus, which output luminescence and fluorescence as a donor and acceptor of BRET, respectively [3]. The construct of the aptamer-protein hybrid would be applicable for sensing various aptamer targets because change in the BRET signal occurs upon steric rearrangement of the protein units upon the interaction of the aptamer and its target. When the target of the aptamer is a small molecule, it may be difficult to cause a large steric rearrangement of the protein units. In such cases, aptamer-based biosensors can also be constructed by optimally locating a small fluorophore, in which fluorescence is sensitive to the microenvironment, close to the aptamer unit. [PERSON] et al. have constructed fluorometric sensors based on a ribonucleopeptide (RNP), which acts as a scaffold to locate RNA aptamer and fluorophore units in proximity.