BIOL 351: Lecture 24 - mRNA Processing

Comprehensive vocabulary flashcards covering eukaryotic post-transcriptional mRNA processing, including 5' capping, 3' polyadenylation, pre-mRNA splicing mechanisms, nuclear export, and mechanisms generating protein diversity.

Primary RNA Transcript (Pre-mRNA)

The nascent, precursor single-stranded RNA produced directly by transcription that must undergo post-transcriptional modifications—including $5'$ capping, splicing, and $3'$ polyadenylation—to become mature mRNA.

$5'$ m7G Cap

A modified guanine nucleotide (7-methylguanosine) added to the $5'$ end of eukaryotic pre-mRNA via an unusual $5'\text{-to-}5'$ triphosphate linkage after the synthesis of the first $20\text{--}30$ nucleotides.

Guanylyl Transferase (GTase)

The nuclear enzyme responsible for catalyzing the addition of a guanine monophosphate to the $5'$ diphosphate end of a nascent pre-mRNA molecule to form the $5'$ cap.

$5'\text{-to-}5'$ Triphosphate Linkage

The distinctive chemical bond connecting the terminal 7-methylguanosine cap to the first transcribed nucleotide at the $5'$ end of eukaryotic pre-mRNA.

eIF4

A eukaryotic translation initiation factor complex that binds directly to the $5'\text{mG}$ cap of mature mRNA to recruit the small ribosomal subunit and initiator tRNA.

Kozak Sequence

A consensus sequence on eukaryotic mRNA (defined as GCCRCCAUGG) that contains the initial AUG start codon scanned and recognized by the small ribosomal subunit during translation initiation.

$3'$ Poly-A Tail

A non-templated tract of approximately $50\text{--}200$ adenine residues enzymatically added to the cleaved $3'$ end of pre-mRNA to stabilize the transcript, promote nuclear export, and facilitate translation.

Cleavage and Polyadenylation Specificity Factor (CPSF)

A cleavage and polyadenylation protein factor that recognizes and binds specifically to the AAUAAA polyadenylation signal sequence located upstream of the pre-mRNA cleavage site.

Cleavage Stimulating Factor (CStF)

A regulatory protein complex that binds to the downstream U-rich or GU-rich sequence of the pre-mRNA to help recruit the cleavage machinery during $3'$ end processing.

Cleavage Factors (CFI and CFII)

Endonucleolytic cleavage factors that assemble with CPSF and CStF to cleave the pre-mRNA transcript $15\text{--}30$ nucleotides downstream of the polyadenylation signal sequence.

Poly-A Polymerase (PAP)

The template-independent enzyme that synthesizes the poly-A tail by sequentially adding adenine nucleotides to the newly cleaved $3'$ end of pre-mRNA.

Poly-A Binding Protein II (PABII / Nab2)

A protein that binds to the nascent poly-A tail to accelerate the rate of adenylation by PAP, control tail length, stabilize the transcript, and assist in nuclear export.

Nuclear Pore Complex

A multiprotein channel spanning the nuclear envelope through which mature mRNA ribonucleoprotein complexes are selectively exported from the nucleus to the cytoplasm.

Exons

Expressed segments of a eukaryotic gene and primary transcript that are retained in the mature mRNA following splicing and encode functional polypeptide sequences or untranslated regions.

Introns

Intervening non-coding sequences within eukaryotic pre-mRNA that are excised by the spliceosome before nuclear export and translation.

$5'$ Splice Site (Donor Site)

The conserved junction located at the $5'$ end of an intron, containing the consensus sequence GURAGU with an almost invariant GU dinucleotide.

$3'$ Splice Site (Acceptor Site)

The conserved junction located at the $3'$ end of an intron that consists of a pyrimidine-rich tract followed by an invariant AG dinucleotide.

Branch Site

A pyrimidine-rich consensus region located $20\text{--}40$ nucleotides upstream of the $3'$ splice site that contains an invariant branch point adenine essential for splicing.

Spliceosome

A large dynamic ribonucleoprotein complex composed of five small nuclear ribonucleoproteins (snRNPs U1, U2, U4, U5, and U6) that catalyzes the precise excision of introns and ligation of exons.

snRNPs (Small Nuclear Ribonucleoproteins)

RNA-protein complexes containing small nuclear RNAs (snRNAs) and associated proteins that recognize conserved splice sequences and assemble to form the active spliceosome.

Lariat Structure

The looped, lasso-shaped intermediate formed during pre-mRNA splicing when the $5'$ end of the excised intron is covalently joined to the $2'\text{-OH}$ of the branch point adenine.

$2'\text{-}5'$ Phosphodiester Bond

The atypical phosphodiester linkage formed between the $5'$ phosphate of the first intron nucleotide (guanine) and the $2'\text{-OH}$ group of the internal branch point adenine during lariat formation.

Alternative Splicing

A regulated post-transcriptional process in which different combinations of splice sites are utilized within a pre-mRNA to yield distinct mature mRNA transcripts and protein isoforms from a single gene.

Alternative Promoters

The presence of two or more distinct transcription start sites ($+1$ positions) within a single gene that generate primary transcripts with differing $5'$ ends in different cell types.

Alternative Polyadenylation

The usage of different polyadenylation signal sequences within a single pre-mRNA to generate mature mRNAs that vary in the length and sequence of their $3'$ untranslated regions or coding regions.