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LncRNA – Long non-coding RNA

As their name suggests, lncRNA molecules are a bit longer transcripts when compared to miRNAs, siRNAs, snoRNAs, etc. (longer than 200 nucleotides) and they do not code for proteins. They are processed in a similar way like mRNA molecules - they have 5’ Guanine cap added, 3’ poly Adenylation site, and some things are spliced out. However, they do not have open reading frames (required for transcription to be actually successful). They are abundant, though. In fact, some research suggests that there are four times more long non-coding RNA molecules than messenger RNAs.

Unlike other small RNA molecules that are usually well conserved, long non-coding RNAs are not. This is mostly due to the fact that they are longer, because the longer the nucleic acid is, the more variation there will be. There are some parts conserved though, like some elements in introns, elements required for splicing, as well as promoters. Another reason why they are not conserved is because they do not code for any protein, which speeds up their evolutionary change comparing to mRNAs.

Although long non-coding RNAs can be present in thousands, only small portion of them are actually functional, which is kind of intuitive considering the fact that they are non-coding. However, there are a lot of processes in which they are involved, like:

- gene-specific transcription – regulation of gene expression along with transcription factors by targeting transcriptional activators, repressors, or some components of RNA polymerase
- regulation of transcriptional machinery – targeting and regulation of initiation complex by forming a stable RNA-DNA triplex along with the promoter
- post-transcriptional regulation – regulation of post-transcriptional mRNA processing by binding to complementary regions and forming duplexes effectively preventing additional molecules to bind there, which can affect pre-mRNA’s processing, transport, translation, etc.
- epigenetic regulation – affecting epigenetic control (DNA methylation, histone methylation and acetylation) by recruiting other proteins

There are a lot of other processes in which long non-coding RNA molecules are involved, like splicing, translation, gene expression regulation by siRNA, genomic imprinting, X-chromosome inactivation, and others…