Figure 3.

How viruses pack maximum information into small genomes. A. Many viruses produce one or more large polyprotiens that are cleaved by cellular or viral proteases co- or posttranslationally, resulting in the generation of multiple viral proteins all of which were transcribed via a single promoter. B. Some viruses utilize the cell’s RNA splicing machinery, which makes it possible to place a single, common promoter in front of different viral genes. C. Some viruses use overlapping reading frames, which saves genetic space. D. Ribosomal frame-shifting is used by retroviruses. If the ribosome slips back a base and then proceeds, the reading frame is shifted and a different protein is produced.