Retroid RNA/DNA Genome Diversity and Genomic Replication Strategies

NB: more material is also available here on retroid elements generally: these include retrotransposons, retroposons and retrons.

Retroid Genome Types:

A) Type VI: diploid ssRNA genomes replicating via longer-than-genome-length dsDNA intermediates

Replication Scheme Diagram

Class VI viruses include:

Only viruses of vertebrates in the family Retroviridae

These are (by genus):

• mammalian type B retroviruses (eg: mouse mammary tumour virus, MMTV)
• mammalian type C retroviruses (Moloney murine leukaemia virus, MuLV)
• avian type C viruses (avian leukaemia virus, ALV)
• type D viruses (Mason-Pfizer monkey virus, MPMV)
• BLV-HTLV viruses (human T-lymphotropic virus, HTLV)
• Lentivirus (HIV, SIV)
• Spumavirus (human spumavirus)

The virus genomes are:

• diploid ssRNA (held together by protein)
• messenger sense
• capped and poly-A tailed
• with 4 basic coding regions:

gag-pro-pol-env

Retroviridae replicate via a dsDNA, longer-than-genome-length intermediate (provirus), which is integrated covalently into the host cell chromosomal DNA.

Conversion of RNA to DNA, and integration into host DNA, is done by the viral-coded RNA-dependent DNA polymerase, which also has RNAse H activity, and DNA-dependent DNA pol activity, as well as encoding the integrase function. Transcription of viral mRNA - which is spliced to allow expression of 3'-proximal ORFs - is by host RNA pol II.

An animation of the initial replication process - as culled from the Leicester material - is shown here; a still image is shown here.

An animation of the infection process - also from the Leicester material - is shown here.

An animation of the HIV infection process is shown here.

(return to Top)

B) Type VII: dsDNA genomes replicating via longer-than-genome-length ssRNA intermediates

Replication Scheme Diagram

Class VII viruses include:

Viruses with encapsidated circular dsDNA genomes (usually gapped or nicked), which replicate via longer-than-genome-length messenger-sense ssRNA intermediates. Such as:

• viruses infecting vertebrates (Hepadnaviridae)
• isometric viruses infecting plants (Caulimoviruses)
• bacilliform viruses infecting plants ("Badnaviruses")
• and now an infectious gypsy-type retrotransposon of Petunia (petunia vein-clearing virus)

The viruses have:

• partially double-stranded non-covalently closed circular DNA, 3.2 kb (Hepadnaviridae)
• circular dsDNA with single-strand break(s) in both strands, about 8 kb (Caulimoviruses and Badnaviruses and petunia virus)

All the viruses are probably derived from the same origin as each other, which they share with Retroviridae: for this reason Class VII viruses have been proposed as "Pararetroviridae". 

 
Back to Contents

Copyright Ed Rybicki, November 1997, August 1998, March 1999, October 2000