Bacteriophage 5′ untranslated regions for control of plastid transgene expression

Expression of foreign proteins from transgenes incorporated into plastid genomes requires regulatory sequences that can be recognized by the plastid transcription and translation machinery. Translation signals harbored by the 5′ untranslated region (UTR) of plastid transcripts can profoundly affect the level of accumulation of proteins expressed from chimeric transgenes. Both endogenous 5′ UTRs and the bacteriophage T7 gene 10 (T7g10) 5′ UTR have been found to be effective in combination with particular coding regions to mediate high-level expression of foreign proteins. We investigated whether two other bacteriophage 5′ UTRs could be utilized in plastid transgenes by fusing them to the aadA (aminoglycoside-3′-adenyltransferase) coding region that is commonly used as a selectable marker in plastid transformation. Transplastomic plants containing either the T7g1.3 or T4g23 5′ UTRs fused to Myc-epitope-tagged aadA were successfully obtained, demonstrating the ability of these 5′ UTRs to regulate gene expression in plastids. Placing the Thermobifida fusca cel6A gene under the control of the T7g1.3 or T4g23 5′ UTRs, along with a tetC downstream box, resulted in poor expression of the cellulase in contrast with high-level accumulation while using the T7g10 5′ UTR. However, transplastomic plants with the bacteriophage 5′ UTRs controlling the aadA coding region exhibited fewer undesired recombinant species than plants containing the same marker gene regulated by the Nicotiana tabacum psbA 5′ UTR. Furthermore, expression of the T7g1.3 and T4g23 5′ UTR::aadA fusions downstream of the cel6A gene provided sufficient spectinomycin resistance to allow selection of homoplasmic transgenic plants and had no effect on Cel6A accumulation.