Such enzymes generally catalyze synthetic reactions unique to viruses, such as synthesis of viral RNA from an RNA template or of viral DNA from an RNA template. More complex viruses may contain other proteins that are important for an efficient infectious cycle. For example, VP16 protein in herpesviruses activates transcription of viral immediate-early genes to initiate the viral program of gene expression. Cellular components . Virus particles can also contain cellular macromolecules that play important roles during the infectious cycle, such as the histones, which help package papovavirus DNA within the virion.
Enveloped viruses can easily incorporate cellular proteins and other macromolecules during budding.
The best example the cyclophilin A in HIV-1 virions. A is not a prerequisite for assembly, particles that lack this cellular chaperone are poorly infectious because they fail to complete some early steps in the infectious cycle.