Engineering chloroplasts for high-level foreign protein expression.

Expression of transgenes from the plastid genome offers a number of attractions to biotechnologists, with the potential to attain very high protein accumulation levels arguably being the most attractive one. High-level transgene expression is of particular importance in resistance engineering (e.g., via expression of insecticidal proteins) and molecular farming. Over the past years, the production of many commercially valuable proteins in chloroplast-transgenic (transplastomic) plants has been attempted, including pharmaceutical proteins (such as subunit vaccines and protein antibiotics) and industrial enzymes. Although, in some cases, spectacularly high foreign protein accumulation levels have been obtained, expression levels were disappointingly poor in other cases. In this review, I summarize our current knowledge about the factors influencing the efficiency of plastid transgene expression and highlight possible optimization strategies to alleviate problems with poor expression levels.