a template for the synthesis of a complementary DNA copy. Reverse transcriptase also has RNaseH activity for destruction of the RNA-template. Integrase binds both the viral cDNA generated by reverse transcriptase and the host DNA. Integrase processes the LTR before inserting the viral genome into the host DNA. Tat acts as a trans-activator during transcription to enhance initiation and elongation. The Rev responsive element acts post-transcriptionally, regulating mRNA splicing and transport to the cytoplasm. Proteome The lentiviral proteome consists of five major structural proteins and 3-4 non-structural proteins (3 in the primate lentiviruses). Structural proteins listed by size: Gp120 surface envelope protein SU, encoded by the viral gene env. 120000 Da (Daltons). Gp41 transmembrane envelope protein TM, also encoded by the viral gene env. 41000 Da. P24 capsid protein CA, encoded by the viral gene gag. 24000 Da. P17 matrix protein MA, also encoded by gag. 17000 Da. P7/P9 capsid protein NC, also encoded by gag. 7000-11000 Da. The envelope proteins SU and TM are glycosylated in at least some lentiviruses (HIV, SIV), if not all of them. Glycosylation seems to play a structural role in the concealment and variation of antigenic sites necessary for the host to mount an immune system response. Enzymes: Reverse transcriptase RT encoded by the pol gene. Protein size 66000 Da. Integrase IN also encoded by the pol gene. Protein size 32000 Da. Protease PR encoded by the pro gene (part of pol gene in some viruses). dUTPase DU encoded by the pro gene (part of pol gene in some viruses), the role of which is still unknown. Protein size 14000 Da. Gene regulatory proteins: Tat: main trans-activator Rev: important for synthesis of major viral proteins Accessory proteins: Nef: negative factor Vpr: regulatory protein Vif: APOBEC3 inhibitor Vpu/Vpx: unique to each type of HIV p6: part of gag Antigenic properties Serological relationships: Antigen determinants are type-specific and group-specific. Antigen determinants that possess type-specific reactivity are found on the envelope. Antigen determinants that possess type-specific reactivity and are involved in antibody mediated neutralization are found on the glycoproteins. Cross-reactivity has been found among some species of the same serotype, but not between members of different genera. Classification of members of this taxon is infrequently based on their antigenic properties. Epidemiology Symptoms and host range: the virus's hosts are found in the orders Primates (humans, apes and monkeys), Carnivora (cats, dogs and other carnivores), Perissodactyla and Artiodactyla