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The minimal genome paradox

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Maedi virus has been isolated from sheep and serological tests, cytopathological data and biochemical examinations confirmed that the isolated virus was the maedi one. Viral particles were visible under an electron microscope in negatively stained samples prepared from the supernatant of infected cell cultures. The size of the particles corresponded to an average size of the maedi virus. The findings confirmed previous clinical, epizootic and serological data with regard to the existence of maedi disease in Poland.
tRNA has been discovered as a factor playing a central role in the translation of genetic information (encoded in DNA and transcribed to mRNA) into amino acid sequences of proteins. However, subsequent studies led to the hypothesis that during evolution, tRNA originated in replication, not translation. Indeed, there are many examples of tRNA-like molecules playing roles in reactions other than translation, including replication of various replicons. In this review, we have focused on functions of tRNA molecules (not tRNA-like structures) outside of their direct roles in translation as factors for a passive transportation of amino acids into a ribosome and deciphering triplets of nucleotides in codons of mRNA. Interestingly, it appears that such tRNA-dependent reactions are effective only when tRNA is uncharged. The most spectacular examples come from bacterial cells and include induction of the stringent control, regulation of transcription of some operons, and control of replication of ColE 1 -type plasmids. Recent studies indicated that tRNA (not only pre-tRNA, shown previously to be capable of self-excision of intron sequences) can be responsible for specific cleavage of another transcript, a ColE I plasmid-encoded RNA I, which is involved in the regulation of plasmid DNA replication initiation. If this reaction is not restricted to RNA I but represents a more general phenomenon, one might suspect a potential role for uncharged tRNA molecules in regulation of various processes, whose efficiency depends on tRNA-cleavable RNAs. This kind of regulation would provide a possibility for a cell to respond to different nutrition conditions resulting in different levels of tRNA aminoacylation.
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