Biosynthesis of the reverse transcriptase of hepatitis B viruses involves de novo translational initiation not ribosomal frameshifting

Abstract

Retroviruses and many other types of genetic elements replicate by reverse transcription of RNA. Although structurally and biologically very diverse, such elements carry conserved polymerase genes (pol) that encode proteins required for reverse transcription. In most cases, the pol gene is preceded by an overlapping gene encoding one or more nucleocapsid proteins, in a different reading frame. Because both coding regions are represented in a single mRNA, the question arises of how the reverse transcriptase in the alternative reading frame is expressed. In retroviruses and retrotransposons it is expressed as a nucleocapsid-polymerase fusion protein by ribosomal frameshifting during translation of the overlapping region. We have examined the mechanism of polymerase biosynthesis in another family of animal viruses that use reverse transcription, the hepatitis B viruses. Genetic and biochemical studies reveal that these viruses do not use ribosomal frameshifting to generate this enzyme, but instead direct translation initiation at an internal initiation (AUG) codon in the polymerase gene.

Functional expression of the leftward open reading frames of the A component of tomato golden mosaic virus in transgenic tobacco plants.

“…Complementation of TGMV 6 suggested that ALI, AL2, and AL3 are translated from the polycistronic RNA in the same transgenic plant. Polycistronic mRNAs have been characterized in other eucaryotes (Kozak, 1986;Sample et al, 1986;Chang et al, 1989). In infected tissue, each of the AL polypeptides may be translated from a different mRNA.…”

Expression of viral polymerase and phosphorylation of core protein determine core and capsid localization of the human hepatitis B virus

“…Either by leaky scanning or by internal initiation, the PG also encodes the viral polymerase (pol) from a start codon downstream of the core AUG (Hwang & Su, 1998;Fouillot et al, 1993;Chang et al, 1989). Pol requires interactions with different heat-shock proteins (at least hsp40, hsp70 and hsp90; Beck & Nassal, 2003;Hu et al, 2004;Stahl et al, 2007) for binding to e. Subsequently the pol-e complex becomes encapsidated into the assembling capsids but only if core is phosphorylated at Ser 162 (Gazina et al, 2000).…”

The sequence between nucleotides 161 and 512 of cowpea mosaic virus M RNA is able to support internal initiation of translation in vitro

“…Nevertheless, we have demonstrated that the 161-512 sequence of CPMV M RNA is sufficient to allow internal entry of ribosomes in vitro, extending the growing list of viral mRNAs capable of supporting this mechanism (Herman, 1986;Hassin et al, 1986;Pelletier & Sonnenberg, 1988;Jang et al, 1988;Curran & Kolakofsky, 1989;Chang et al, 1989) to the plant kingdom.…”