View/Open - TWAS & OWSD Thesis Repository Home

hydrophilicity and antigenicity. However, since the three-dimensionalstructure or antigenic function of the NSS protein has not been determined,the relative contribution of these changes to infectivity and immunogenicityisunknown (Leitmeyeret. al., 1999).2.3.8.4 Significance of NTR difference: The NTRs of several positive-strand RNAviruses are predicted to fold into stem-loop structures that interact with viralor cellular proteins (Day et al., 1992; Andino et al., 1993) binds to the 3'terminus of the minos strand to initiate transcription of the positive strands,thus regulating replication. Mutations that modify these structures, andthereby alter the RNA-protein interactions, have been shown to affectvirulence or cause attenuation (lizuka, 1989;Kinney et al., 1993). Engineeredmutations and deletions in the S'NTR of a full-length DEN-4 cDNA clonewere shown to restrict DEN-4 growth in cell culture and inoculatedmosquitoes (Cahour et al., 1995). Most mutations within thc long stemstructure were lethal, but RNA transcripts containing deletions in the loop orshort-term regions were usually infectious. However, a mutant bearingdeletions in the 3'-terminal loop was least efficient in translation. It has beenshown that an Avto-U mutation that distinguishes South East dengue -2 fromAmerican genotype viruses at position 69 was predicted to change thesecondary structure of the viral RNA: a 4-nt long bulge was formed at the S'terminus of the American genotype, which reduced the length of the stem,but increase the length of the 3' -tenminalloop. Whether the presence of thissmall bulge could reduce translation efficiency remains unclear. Bulges assmall as 1nt have been shown to reduce RNA -protein interactions in otherviral systems (Weber and Konisberg, 1975; \Vu and Uhlenbeck, 1987)The 3' NTR contains sequences essential for virus replication andgrowth, serving as signals for the initiation of minus -strand synthesis, andpossiblypackaging in YF and DEN-4 (Hahn et al., 1987; Men et al., 1996).A stable secondary structure motif, formed by the 3' terminal 100nt, wasdescribed for all, mosquito-borne Flaviviruses studied to date (Rauscher etal., 1997; Shi et al., 1996). Within this region and farther upstream, threehighly conserved sequences termed CS1, CS2, and RCS2, are thought to be37