MicroRNAs are small RNA molecules that bind to the 3^' end...

MicroRNAs are small RNA molecules that bind to the $3^{\prime}$ end of mRNAs and suppress trandation (see Chapter 14). How miRNAs suppress translation is still being investigated. Some eukaryotic mRNAs have internal ribosome-binding sites downstream of the $5^{\prime}$ cap, where ribosomes normally bind. In one investigation, miRNAs did not suppress the translation of ribosomes that attach to internal ribosome-binding sites (R. S. Pillai et al. 2005. Science $309: 1573-1576$ ). What does this finding suggest about how miRNAs suppress translation?

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Key Concepts

internal ribosome entry sites (IRES)

Internal ribosome entry sites are sequences within the mRNA that allow ribosomes to bind and initiate translation independently of the 5? cap. This cap-independent mechanism bypasses the typical initiation factors required for cap-dependent translation, demonstrating that if miRNAs specifically disrupt cap-dependent processes, they might not affect translation via IRES.

microRNAs

MicroRNAs are short, non-coding RNA molecules that play a regulatory role in gene expression. They bind to messenger RNAs (mRNAs), typically at the 3? untranslated region, and can suppress protein synthesis either by degrading the mRNA or by inhibiting translation. Their function is a key aspect of post-transcriptional regulation.

cap-dependent translation initiation

Cap-dependent translation is the primary mechanism by which ribosomes are recruited to mRNAs for protein synthesis. It involves the recognition of the 5? cap structure on the mRNA by the initiation machinery. Any interference with this process, such as by molecules that prevent cap recognition, can inhibit overall translation, which is one of the ways miRNAs are thought to function.

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