Regulation of Expression Occurs at Multiple Steps

Cells can precisely regulate gene expression at every step, from DNA to protein. This regulation occurs during transcription; during RNA processing, localization, and degradation; and during and after translation. 

Transcriptional regulation is mediated by proteins that bind to regulatory sequences on DNA. These transcription factors are one of the most common ways to control gene expression and can either initiate transcription or prevent it.

Transcription generates pre mRNA that must be processed to mature mRNA through several regulated processes. 

mRNA splicing, which removes the non-coding regions in precursor mRNA and joins the coding ones, controls gene expression through differential splicing patterns and RNA binding proteins.  The addition of a poly-A tail and a 5’ cap to produce the mature mRNA is also controlled.

Next, the mRNA must associate with RNA binding proteins to form a complex known as a ribonucleoparticle. This process is highly regulated, and only an mRNA that exists in a ribonucleoparticle can be transported from the nucleus to the cytoplasm to be translated.

Translational control is another crucial point for the regulation of gene expression. Regulation can be specific, where an individual or a subset mRNAs are involved, or general, where most mRNA transcripts are affected. 

In specific regulation, inhibition of translation is controlled through interactions with trans-acting elements, such as proteins and certain types of RNA, including microRNAs and short interfering RNAs.

In general regulation, the various proteins involved in the translation machinery are activated or inhibited, which affects all transcripts.

Finally, post-translational modifications, such as phosphorylation, can either activate or deactivate proteins, while others, like ubiquitination, can lead to their degradation.