Date of Award
Doctor of Philosophy
Molecular Biology, Microbiology and Biochemistry
Gupta, Dr. Ramesh
Ribosomal RNAs (rRNA) undergo several post-transcriptional modifications inside the cell. These modifications can be (1) RNA- independent (enzyme only) and (2) guide RNA-mediated. In the latter mechanism, a group of small, metabolically stable, non-coding RNAs, present as ribonucleoprotein (RNP) particles, modify ribosomal RNAs inside the cell. One of the highly abundant rRNA modifications is pseudouridine (Y) formation. In Archaea and Eukarya, pseudouridine synthases, with the help of small RNAs, form pseudouridines at functionally important regions in rRNA. Cbf5, the pseudouridine synthase, three other core proteins, and a box H/ACA RNA form the ribonucleoprotein complex in sRNP-mediated rRNA pseudouridylation. Certain Ys in rRNAs are evolutionarily conserved from Bacteria to human. Among those, two Ys are present in helix 69 of rRNA and one in helix 90. We successfully deleted Cbf5 in Haloferax volcanii, a haloarchaeon, and showed that the deleted strain was viable. It was the first report where Cbf5 deletion was achieved, because deletion or mutation of cbf5 or of its homologs is lethal in eukaryotes. We also found that the cbf5 deleted strain was unable to produce the three highly conserved Ys in rRNA of H. volcanii (position 1940, 1942 in helix 69, and 2605 in helix 90), whereas the tRNA Ys were intact. To identify the specific structural features of Cbf5 involved in rRNA Ψ formation, we used a cbf5 deleted strain which was complemented with a plasmid borne copy of the gene. Using the crystal structure of Pyrococcous furiosus Cbf5 as template, we created a homology model of H. volcanii Cbf5 (HvCbf5) and identified several residues and motifs/domains of HvCbf5 that might be important to the protein's enzymatic activity. By using an in vivo mutational approach, we confirmed some previously predicted and certain unidentified residues/motifs/domains that serve as positive determinants of rRNA Ys1940, 1942, and 2605 formation inside the cell. A box H/ACA RNA, sR-h45, was bioinformatically predicted before. We confirmed its presence as a double hairpin RNA inside the cell whose level goes down in the absence of Cbf5. We identified that sR-h45 is the guide RNA for sRNP-mediated Ys at the three above mentioned rRNA positions in H. volcanii. Each hairpin of this RNA can independently modify the substrate, both in vivo and in vitro. To characterize the structure of sR-h45, we have used a sR-h45 deleted strain where the function of sR-h45 was complemented with a plasmid-borne copy of the gene. By a combination of in vivo and in vitro mutagenic approaches, we determined specific nucleotides/structures of this RNA, involved in binding to the core proteins and also to the substrate RNA. We also identified that one hairpin of sR-h45 can modify two successive positions (1940 and 1942) in rRNA.
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