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In retroviruses, there are a number of virally encoded proteins essential for 
the replicative process that can be provided in trans by a helper virus. 
These proteins are coded for by three retrovirus genes found in all replication- 
competent viruses, regardless of their species of origin. These genes, the 
gag, pol , and env genes, code for proteins involved respectively in: virion 
encapsidation and assembly, reverse transcription of viral DMA and integration, 
and virion adsorption/penetration. 
Sequences essential in cis for viral gene expression are: the LTRs which 
include regulatory sequences for viral RNA synthesis as well as integration 
sites for insertion of the virus into chromosomal DNA (attachment sequences); 
the sites involved in the initiation of plus and minus strand reverse tran- 
scription; and the primer binding sites (PBS and PPT) utilized in reverse 
transcript ion. 
Although much has been learned about the regulatory features of retroviruses, 
uncertainties remain. Those features of the proviral structure that are thought 
to be necessary for transcription and transmission of the viral genome are: 
the LTR sequence on each end containing regulatory signals for initiating and 
terminating transcription, sequences required for reverse transcription and 
others for proviral integration; short sequences immediately adjacent to each 
LTR and necessary for reverse transcription; the packaging sequence necessary 
for the viral RNA to be packaged into an infectious viral particle; and the 
donor and acceptor splice sites. 
Retroviruses have been isolated from many different classes of vertebrates. 
However, most clases of retroviruses have a fairly limited host range. The 
most common group of mouse retroviruses, ecotrcpic murine leukemia viruses 
(MLV), replicates well only in mouse and rat cells; and the most common species 
of avian retroviruses, avian leukosis viruses, replicates only in chicken and 
quail cells. But this limited host range is not characteristic of all 
retroviruses. Another group of mouse retroviruses, amphotrqpic murine leukemia 
viruses, replicates well in cells from several other mammalian species as well 
as mouse, and another species of avian retroviruses, reticuloendotheliosis 
virus (Rev), replicates well in all avian cells tested and in dog cells. 
The primary determinant of retrovirus host range appears to be the envelope 
glycoproteins. For example, there are several subgroups of avian leukosis 
viruses with different host ranges on chicken cells of different genotypes. 
These avian leukosis virus subgroups differ only in their env gene, the gene 
that codes for the virion glycoproteins. Similarly, there are at least 
three subgroups of mouse leukemia viruses, eco-, xeno-, and amphotropic with 
different host ranges on mammalian and avian cells corresponding to their 
different env genes. 
Other controls on retrovirus host range exist like the effect of the Fv-1 
gene on mouse leukemia virus integration and the lack of processing of Rous 
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