Date of Award
Doctor of Philosophy
Molecular Biology, Microbiology and Biochemistry
In all the three domains of life, most RNAs undergo post transcriptional modifications both on the bases as well as the ribose sugars of the individual ribonucleotides. 2'-O-methylation of ribose sugars and isomerization of Uridines to Pseudouridines are two most predominant modifications in rRNAs and tRNAs across all domains of life. Besides 2'-O-methylation of ribose sugars, methylation of pseudouridine (Ø) at position 54 of tRNA, producing m1Ø, is a hallmark of many archaeal species but the specific methylase involved in the formation of this modification had yet to be characterized. A comparative genomics analysis had previously identified COG1901 (DUF358), part of the SPOUT superfamily, as a candidate for this missing methylase family. To test this prediction, the COG1901 encoding gene, HVO_1989, was deleted from the Haloferax volcanii genome. Analyses of modified base contents indicated that while m1Ø was present in tRNA extracted from the wild-type strain, it was absent from tRNA extracted from the mutant strain. Expression of the gene encoding COG1901 from Halobacterium sp. NRC-1, VNG1980C, complemented the m1Ø minus phenotype of the ÄHVO_1989 strain. This in vivo validation was extended with in vitro tests. Using the COG1901 recombinant enzyme from Methanocaldococcus jannaschii (Mj1640), purified enzyme Pus10 from M. jannaschii and full-size tRNA transcripts or TØ-arm (17-mer) fragments as substrates, the sequential pathway of m1Ø54 formation in Archaea was reconstituted. The methylation reaction is AdoMet-dependent. The efficiency of the methylase reaction depended on the identity of the residue at position 55 of the TØ-loop. The presence of Ø55 allowed the efficient conversion of Ø54 to m1Ø54, whereas in the presence of C55 the reaction was rather inefficient and no methylation reaction occurred if a purine was present at this position. These results led to renaming the Archaeal COG1901 members as TrmY proteins. Another aim of this study was to investigate the mechanism of target RNA recruitment to a box C/D sRNP. From data obtained, we have made the following hypothesis- aNop5p, either alone or as a heterodimer with Fibrillarin, binds to single stranded bulges and loops of target RNA. This aNop5p bound target is then hybridized to an assembling guide sRNP complex containing the guide RNA and L7Ae or guide RNA, L7Ae and aNop5p. If the guide:target sequences are complementary to each other, they hybridize and the target nucleotide gets modified. We also think that post modification, the guide and target strands separate, the core proteins rearrange themselves on the guide RNA and then prime the guide RNA for next round of modification. Compared to the general archaeal populations, haloarchaea contain significantly fewer number of box C/D guide RNAs. In archaea, previous studies have underscored the importance of a symmetric assembly of the core proteins on the sRNA. This meant that if the core proteins were unable to bind to either the terminal box C/D or the internal box C'/D' motifs, the sRNP was not efficient to carry out the modification of the target RNA. Essentially the only two haloarchaeal box C/D sRNPs known before had a symmetric architecture. In this study we discovered the first naturally occurring asymmetric box C/D sRNP called sR-41 in Haloferax volcannii. The architecture of Haloferax volcanii sR-41 box C/D sRNP seems to be closer in conformation to eukaryal snoRNPs (eukaryal counterparts of archaeal sRNPs) in which the core proteins assemble asymmetrically on the RNA. Till date, no information regarding the catalytic mechanism of an asymmetrically arranged eukaryal box C/D snoRNPs are available, because of unavailability of any assembly systems or crystal structures. Hence, this archaeal sR-41 guide sRNP provides a unique opportunity to study mechanism of modification in an asymmetrically arranged box C/D sRNP molecule.
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