When trying to express a mammalian gene in a bacterium, expression vectors are o

One issue with transcription of the cloned gene(s) is the differences in RNA polymerases and their recognition (promoter) sequences. For example, strong eukaryotic promoters might not function inside a bacterial cell. Another problem is that origins of replication in vectors are not universal to hosts, meaning that plasmids that replicate in eukaryotes cannot necessarily replicate in E. coli. Most importantly, a fundamental difference in expressing eukaryotic genes in prokaryotes is that prokaryotes generally lack the machinery required to splice out introns.