Human Immunodeficiency Virus (HIV) is a retrovirus that causes Acquired Immune Deficiency Syndrome (AIDS). HIV infection causes a progressive failure of the immune system, causing cancers and various infections to overtake the patient.
HIV is present as both free virus particles and virus within infected immune cells and is transmitted via blood products (contaminated needles, unscreened blood), mother to child (pregnancy, labor, nursing) and unprotected sex. The life cycle of a retrovirus is illustrated below.
What enzyme is needed for the virus to proceed from step 4 to step 5?