Lecture B4 - Transcription and RNA Processing in Trypanosoma. Flashcards
What are the 2 major switches in the T brucei life cycle?
Slender to stumpy bloodstream form
Bloodstream form to pro cyclic (tse tse midgut form)
What do variant surface glycoproteins do?
Encode for the coats of the trypanosomes - essential for their survival and avoiding the immune system.
Describe how genes are organised in trypanosomes.
Organised in tenderly repeated gene families - housekeeping proteins and developmentally regulated proteins.
What is polycistronic transcription?
Operons and genes are transcribed together.
How is the 5’ end and 3’ end formed?
5’ end cap provided by trans-splicing.
3’ end formed by polyadenylation.
What is the evidence for polycistronic transcriptional units in trypanosomes?
Calmodulin gene is repeated in 4 copies.
Special inhibitors of processing used that interferes with the processing of the mRNA to find mRNA molecules that are the size of 1, 2, 3 or 4 genes providing evidence that the mRNA is transcribed as a 4 gene mRNA that is then rapidly processed into a single copy.
What is cis-splicing?
Cis-splicing is the process that removes the introns from mRNAs of eukaryotic genes. Splicing is accomplished by a complex of small nuclear proteins and RNAs spliceosome.
Removal of introns precisely (to a single bp).
What is trans-splicing?
Happens in trypanosomes.
The 39 first base pairs of all trypanosome mRNAs is identical and this sequence is not found in the genomic locus of these genes.
Individual mature mRNAs are derived from large polycistronic transcripts by a process called trans-splicing.
In this process mRNAs for individual genes are cut out of the polycostronic transcript and a short RNA transcribed from a different locus is attached to its 5’ end.
What are the similarities of cis splicing and trans splicing?
Small nuclear RNPs
What is the difference between cis and trans splicing?
Cis splicing eliminates introns, only poly(A) polymerase has introns in trypanosomes.
Trans splicing adds the spliced leader to mRNA exon sequence, first observed in trypanosomes, later found in worms and euglena.
What are some common features of trypanosomes mt transcripts?
U residues suppress internal reading frame shifts.
A ‘U’ G initiation codon
‘U’ XY stop codons
What do the U codons do?
Mature mRNA can see where the U bases have been added or deleted to give the correct coding sequence.
Describe guide RNA.
Complementary to edited RNA sections if G U is allowed.
Small (40-70 nt).
Guide because they are not conventional templates.
Structural elements - anchor, informational part and Oligo(U)tail.
Guide RNA knows where to go as it has the sequence that matches up. Anchor machinery recognises the difference between the sequences and can add or delete the bases as appropriate.
What adds and deletes uracil?
Tutase - adding uracil
Exouase - deleting uracil
