Discrepancias políticas y propuestas de gobierno regional

Retroviral particles (virions) range from 80-13Onm in diameter, and contain two identical copies of a plus-sense single stranded RNA genome complexed with viral- coded proteins from gag and pol genes. For the simplest retroviruses such as MLV, the genome is between 7 and lOkb in length. Retroviral virions also contain small RNA and DNA molecules which are probably incidental, and a specific tRNA unique to related family members, which primes RNA-dependant DNA synthesis. All retroviruses encode

gag, pro (protease), pol, and env genes in this order, with gag and gag-pro-pol proteins synthesized from a full length mRNA species, and env protein synthesized from a spliced sub-genomic species (Fig 4.1). The nucleoprotein complex is surrounded by a host cell-derived lipid bilayer envelope which incorporates glycoprotein encoded by the

env gene.

Retroviral gag genes encode polyproteins that are cleaved into at least three proteins designated matrix (MA), capsid (CA), and nucleocapsid (NC), a process which is mediated by products (PR) o f the protease gene during virion assembly. Gag proteins constitute the major structural elements o f the viral capsid. MA proteins line the inner surface o f the envelope and are essential for membrane association and budding. CA

[A]

R U5 U3 R

SDe gag-pro-pol SAe en v

PB -

t

T

CCAAT TATA PolyA

U3

e n h a n c e r e le m e n ts

Fig 4.1. Retrovirus RNA genome and synthesis of full length and spliced MLV retroviral RNA. [A] Schem atic o f full length M LV R N A gen om e, sh ow in g replication primer binding sites (P B ), sp lice donor and acceptor sites for env m essage (S D e, S A e). The position o f the minim al packaging seq u en ce is represented by v|/. The arrangement o f the open reading frames are described in the text. [B ],[C ] The integrated M LV provirus acts as a tem plate for transcription o f viral R N A , w hich is m ediated by cellular R N A polym erase II, and regulated by long terminal repeat sequences (LTR) formed during reverse transcription from the RNA genom e. For M L V , polyadenylation is active in the 3 ’ LTR but suppressed in the 5 ’ LTR. The full length transcript is the precursor transcript for g a g -p r o -p o l m essage, subgenom ic en v transcripts, and gen om ic R NA . For M LV, the en v splice donor site (S D e) is upstream o f gag initiation codon.

forms the core shell of the virion, and NC rests in intimate association with genomic RNA. In MLV, the pro gene is placed after the stop codon in gag but in the same reading frame as pol. Both pro and pol genes are expressed by in-frame readthrough facilitated by suppression of the termination codon at the end o f gag by a glutamine tRNA. Reverse transcriptase (RT) is an RNA-dependent DNA polymerase encoded by

pol which binds to the tRNA primer, and which in a distinct domain possesses RNaseH activity. Integrase (IN) is a separate protein derived from the carboxy terminus of the

p o l gene. The env gene encodes a polyprotein that is post-translationally modified by host cell enzymes to yield a transmembrane domain (TM), and an external glycosylated surface domain (SU) which determines the host range of the virus, and contains the major determinants for recognition by neutralising antibodies. TM anchors SU to the membrane, and mediates fusion of virus and host cell membranes. In membranes, SU and TM form complexes of 2-4 heterodimers each o f which is stabilised by disulphide linkages (Avian sarcoma-leukosis virus, ASLV), or non covalent interaction (MLV).

The single stranded RNA genome consists of an internal region containing viral open reading frames, and flanking regions which are necessary in cis for replication of the virus (Fig 4.1 A). The 5’ end o f virion RNA and viral mRNA is capped and methylated, and the 3’ ends are polyadenylated. These modifications are undertaken by host cell enzymes. A short repeated sequence R is located at each end of the genome and mediates DNA strand transfer during reverse transcription. Another short sequence U5, is located between R and the minus-strand primer-binding site (PB-) at the 5’ end of the genome, and is necessary for formation of secondary stmctures which guide efficient initiation o f reverse transcription. PB- is complementary to 16-19 bases at the 3’ end of the specific tRNA molecule. Following PB is an untranslated leader sequence which precedes the initiation codon for gag and which incorporates a packaging sequence (v|/) which for MLV is about 300bases long, and lies immediately downstream from the env

splice donor site (SDe). Subgenomic env mRNA is therefore not assembled into virions because the cis-acting signal is in the env intron (Fig 4 .IB). At the 3’ end of the genome, downstream of env a short purine rich sequence (polypurine tract, ppt) initiates synthesis o f plus-strand DNA (PB+). U3 is unique to the 3’end o f the RNA genome, but

is duplicated in the proviral genome, and contains elements necessary for transcriptional regulation of the integrated provirus (Fig 4.1C).