Chemically Modified Oligonucleotides as Probes and Inhibitors

Oligonucleotides bind specifically to single-stranded nucleic acids to form a double helix if there is a complementary antiparallel nucleotide sequence. In addition, certain oligonucleotides bind specifically to a variety of proteins. Therefore, biological processes involving these nucleic acids or proteins can be modulated (normally inhibited) by addition of the respective oligonucleotides. The effects of these oligonucleotides and the fields of potential application can be broadened by the introduction of chemically modified nucleotides. For instance, replacing oligonucleotide phosphate groups by methylphosphonates results in the loss of one negative charge per nucleotide and the oligonucleotide becomes more lipophilic. An oligonucleotide carrying a reactive group can modify its binding partner. An oligonucleotide covalently linked to a dye can be localized in a biological specimen. Oligonucleotides attached to an enzyme can be detected in very small amounts since the enzyme can catalyze the formation of large amounts of the substance assayed (e.g., a fluorescent product). An important biochemical application is the detection and localization of a messenger RNA or its gene. Medical applications include the detection of bacterial or viral sequences. There is also great interest in inhibiting the translation of messenger RNA and the transcription and replication of DNA. So-called antisense oligonucleotides are currently being used for the inhibition of protein biosynthesis and of reverse transcription of retroviruses.