T.14 Tipos de procesamiento del RNA / RNPs en la célula eucariota

Types of RNA Processing

• RNA undergoes modification during or after transcription, which is essential for understanding its function
• There are three subgroups of RNA:
  • RNA that carry information (mRNA)
  • RNA tools (tRNA, rRNA, snRNA)
  • Non-coding RNA (not studied in detail)

Modifications of RNA

• Modifications can be classified by type and in which RNA molecule they occur
• Modifications can occur in both prokaryotes and eukaryotes
• The table shows modifications of RNA precursors, with the last column added later to show the formation of RNP, which is essential for RNA function

RNPs in Eukaryotic Cells

• Ribonucleoproteins (RNPs) are stable associations between proteins and RNA
• In eukaryotic cells, RNA is never bare, but associates with proteins to protect it from degradation and for recognition
• RNPs are formed in the nucleus before transporting to the cytoplasm
• Proteins can exchange with others in the cytosol, resulting in different proteins bound to the RNA

mRNPs in Eukaryotic Nucleus

• The association of RNA with proteins provides information about its fate
• RNA modifications can be read by proteins, which then bind to the RNA
• The code is written in the RNA as chemical modifications, which are read by proteins and specific binding occurs
• Readers of the code include degradation machinery, transport machinery, spliceosomes, and translation machinery

Capping of RNA

• The cap is a structure that associates exclusively with RNA transcribed by RNA polymerase II
• If the cap is not added, the primary transcript is rapidly degraded
• The cap is a guanine nucleotide that is "in reverse" and is resistant to degradation
• The process of capping involves phosphatases, guanylyltransferases, and methyltransferases

Role of CTD

• The enzymes involved in capping are attracted to the CTD tail of RNA polymerase II, which is phosphorylated on serine 5
• The binding of enzymes to the CTD tail is important, as it accelerates the reaction rate
• The enzymes are released once the serine 2 is phosphorylated

NAD Capping

• Recently discovered: NAD capping in mRNAs, where NAD is a dinucleotide (A linked to nicotinamide)
• NAD is added to the end of a messenger by a phospho-phospho bond
• In prokaryotes, NAD is not added later, but RNA polymerase uses NAD as the first nucleotide instead of ATP
• The modification is a signal for the cell to read and degrade the RNA in a specific way

Poly (A) and Poly (U) Tails

• Poly (A) tails are found in the other end of transcripts, and are only introduced in messengers (except for histone messengers)
• The introduction of poly (A) tails is a mechanism of transcription termination
• Poly (U) tails are introduced by the TUTase enzyme, which is a writer that marks RNA for degradation
• Poly (U) tails can be introduced in RNA that did not have a tail previously, such as histone mRNAs
• The degradation of RNA with poly (U) tails is dependent on uridinilation