PubMed · 3187491
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T M Beardsley. 1988. Supertests.. https://doi.org/10.1038/scientificamerican0288-34
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Infection with hepatitis B virus can result in asymptomatic seroconversion with viral clearance, fulminant hepatic failure and death, or chronic, typically lifelong, transmissible infection. The mechanism(s) of viral persistence are poorly understood but viral clearance and fulminant hepatic failure are generally thought to result from co-ordinated and effective and abnormally vigorous immune responses, respectively, whereas viral persistence results from immunological failure in addition to poorly characterized viral factors promoting persistence. This paper proposes (1) that the predominant viral factor(s) promoting persistence of hepatitis B virus are homeostatic mechanism(s) responsible for modulating its replication and mutation and (2) that chronic hepatitis B results when these mechanisms are successful and other outcomes occur when these homeostatic mechanism(s) fail. Furthermore, it is proposed that seroconversion (e.g. from HBsAg to anti HBsAg positivity), when it occurs, is a consequence facilitated by restricted viral antigenic diversity and reduced viral replication rather than a proximate cause of it. The specific homeostatic mechanisms proposed--negative feedback inhibition of hepatitis B virus DNA polymerase/reverse transcriptase mediated by HBs antigen and a hepatitis B virus DNA polymerase fidelity modulating function of HBeAg--are consistent with the available data and resolve many paradoxical clinical observations. But, more importantly, this model has clear implications for therapy, including the rational design of drugs and therapeutic vaccines.
The rakkyo strain (TMV-R) and the common strain (TMV-U1) of tobacco mosaic virus exhibit distinct host range differences. TMV-R infects rakkyo plants, a monocot host that TMV-U1 is unable to infect. However, TMV-R causes only latent infection in Nicotiana tabacum cv. Bright Yellow (BY) in inoculated leaves, whereas TMV-U1 infects BY systemically and induces mosaic symptoms. Complete nucleotide sequencing of the TMV-R genomic RNA revealed amino acid changes in the 130K/ 180K replicase proteins, the 30K protein, and the coat protein and nucleotide changes in the 5' and 3' noncoding regions compared to TMV-U1. To identify viral components involved in determination of differences in host range, we have mapped determinants for the differential infection phenotype in BY plants by constructing chimeric viruses between the two strains in the present study. Examination of the infection phenotypes of the 14 constructed chimeric viruses in BY showed that determinants defining the differential infection phenotype in BY reside in the 130K/180K replicase proteins and the 3' noncoding region. Cognate combination of the 130K/180K replicase proteins and the 3' noncoding region of TMV-U1 origin is required to produce systemic infection in BY plants.
The ability to selectively amplify RNA in the presence of genomic DNA of analogous sequence is cumbersome and requires implementation of critical controls for genes lacking introns. The convenient approaches of either designing oligonucleotide primers at the splice junction or differentiating the target sequence based on the size difference obtained by the presence of the intron are not possible. Our strategy for the selective amplification of RNA targets is based on the enzymology of a single thermostable DNA polymerase and the ability to modulate the strand separation temperature requirements for PCR amplification. Following reverse transcription of the RNA by recombinant Thermus thermophilus DNA polymerase (rTth pol), the resulting RNAxDNA hybrid is digested by the RNase H activity of rTth pol, allowing the PCR primer to hybridize and initiate second-strand cDNA synthesis. Substitution of one or more conventional nucleotides with nucleotide analogs that decrease base stacking interactions and/or hydrogen bonding (e.g. hydroxymethyldUTP or dITP) during the first- and second-strand cDNA synthesis step reduces the strand separation temperature of the resultant DNAxDNA duplex. Alteration of the thermal cycling parameters of the subsequent PCR amplification, such that the strand separation temperature is below that required for denaturation of genomic duplex DNA composed of standard nucleotides, prevents the genomic DNA from being denatured and therefore amplified.