systems used for expressing recombinant proteins

bacteria (e coli)

features: fast growing, high yield, easy storage, well understood regulatory sequences (lac operon, induced by IPIG/lactose, T7 promoter)
limitations: lack eukaryotic post translational modifications; prone to improper folding or insoluble inclusion bodies
best for: simple non-glycosylated proteins

yeast (S. cerevisiae, P. pastoris)

features: unicellular eukaryotic system offering tight control (GAL1/GAL10 promoters turned ON by galactose, OFF by glucose)
limitations: glycosylation patterns differ from mammalian cells
best for: proteins requiring basic eukaryotic post translational modifications and proper folding

insect cells (baculovirus/Sf9, Sf21)

features: eukaryotic machinery capable of complex post translational modifications and proper folding of large multi-subunit complexes
limitations: more expensive and complex than yeast or bacteria
best for: larger eukaryotic enzymes, receptors. and complex proteins

mammalian cells (HEK293, CHO)

features: provides authentic human like post translational modifications and correct structural folding
limitations: lower yield, slower growth, higher cost
best for: complex human therapeutic proteins and antibodies

transgenic animals

features: recombinant proteins produced in whole organisms (tissue specific expression in cow/goat milk or chicken eggs