An expression vector is used to express cloned DNA fragments in a host cell. It contains regulatory sequences like promoters and enhancers that efficiently transcribe the insert gene. Common elements in expression vectors include an origin of replication, selective markers, promoters, a multiple cloning site, and a terminator. Escherichia coli is widely used as a prokaryotic expression host due to its fast growth, inexpensive media, and ability to produce both prokaryotic and eukaryotic proteins. The lac and tac promoters are commonly used in E. coli expression vectors as they allow for inducible expression of target genes.