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#mRNA maturation

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Processing of mRNA, rRNA, tRNA

Intron removal from pre-mRNA is called

Removal of noncoding introns from precursor messenger RNA and ligation of coding exons is termed splicing, essential step generating continuous open reading frame for translation. Carried out by spliceosome, dynamic ribonucleoprotein complex assembling on consensus signals GU at five prime splice site, branch point adenosine near three prime end, and AG at three prime splice site. Two transesterification reactions without ATP for chemistry, ATP used for helicase remodeling, excise intron as lariat and join exons. Alternative splicing produces multiple isoforms from single gene expanding proteome complexity in metazoans significantly.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 6: Intron removal called splicing; Lodish 9th ed., Pre-mRNA splicing overview

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Processing of mRNA, rRNA, tRNA

Poly(A) polymerase is

Polyadenylate polymerase is classified as template-independent terminal nucleotidyltransferase, lacking ability to read DNA or RNA template for base selection. Instead specificity arises from intrinsic preference for ATP binding pocket and interactions with CPSF and PABPN1. Therefore polyadenine sequence is not encoded in genome but added post-transcriptionally forming uniform adenine homopolymer essential for stability. This contrasts with DNA polymerases, RNA polymerases that are template dependent. Other template independent polymerases include CCA-adding enzyme for transfer RNA and terminal deoxynucleotidyl transferase TdT in immunoglobulin gene rearrangement adding N-nucleotides.

Ref: Berg et al., Biochemistry, 9th ed., Chapter 28: Poly(A) polymerase template independent; Lodish 9th ed., Terminal nucleotidyltransferase family

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