L4: Eukaryotic rRNA & tRNA

Transcription in Eukaryotes

Three main types of nuclear RNA polymerases

1. RNA polymerase I: rRNAS

2. RNA polymerase II: mRNAs, telomerase RNAs

3. RNA polymerase III: tRNAs

4. RNA polymerase IV and V (plants only): siRNAs

Transcription factors

Regulate activity of RNA polymerases

Newly transcribed RNAs are processed

Primary transcript (pre-RNA)

Initial RNA molecule synthesized (equivalent in length to corresponding DNA sequence - the transcription unit)

Small RNAs are required for RNA processing into functional RNA

Percent of rRNA in cells

80

rDNA

DNA sequence encoding rRNA that are repeated hundreds of times (moderately repeated units)

Where are ribosomes produced?

In nondividing cells, rDNA are clustered in the nucleoli, where ribosomes are made

Ribosomes

Consist of large and small subunits

• Ribosomal proteins and rRNAs

• Prokaryotic ribosomes are 70s (S for Svedberg cof.)

• Eukaryotic are 80s

Synthesizing the rRNA Precursor

Proteins are associated with pre-rRNA before transcription

The non-transcribed spacer separates transcription units in a ribosomal gene cluster

Processing rRNA precursor

A single primary transcript (pre-rRNA) can be cleaved into three rRNAs: 28S, 18S, and 5.8S

Pre-rRNA contains large numbers of methylated nucleotides and pseudouridine residues (for stability, assembly, translation)

Unaltered sections of pre-rRNA are discarded

RNA polymerase III synthesizes what rRNA?

5S

Location: outside nucleolus

Via: internal promoter

snoRNAs

Processing of pre-rRNA and nucleolar RNAs

can modify bases in pre-RNAs

snoRNPs (small nucleolar ribonucleoproteins)

what snoRNAs are packaged into with proteins

Bound snoRNA (methylase or pseudouridylate) guide enzymes with snoRNPS to modify a nucleotide in pre-rRNA)

200 different snoRNAs, one for each site in the pre-rRNA that is ribose-methylated or pseudoridylated

Transfer RNA (tRNA)

Transfer RNAs are synthesized from genes that are found in small clusters (tDNA)

One cluster can contain multiple copies of different tRNA genes

tRNAs are transcribed into which type of RNA polymerase?

RNA polymerase III, so tRNAs have promoter sequences within the coding region of gene

tRNA processing

tRNA precursor is trimmed and numerous bases must be modified

tRNA properties

Small and stable (75-90 nucleotides)

Contain post-transcriptionally modified bases

tRNAS have a cloverleaf structure