Control of Gene Expression, Posttranscriptual Regulation

Posttranscriptional gene regulation is a complex, multistage phenomenon that helps to fine-tune the levels of active proteins in the cell. Transcription is followed by RNA processing to produce a mature transcript for translation. The stability of this transcript and the efficiency of translational initiation are important regulatory steps, particularly in the chloroplast genome. Noncoding small- and micro-RNA molecules play key roles in regulating gene expression by identifying and binding with mRNA transcripts that are targeted for silencing. After protein synthesis, the protein must be folded and chemically modified, processes that depend largely on the site of accumulation. In transgenic plants, all stages of transgene expression can be controlled by expression construct design, but other factors are involved that are influenced by the integration of the transgene and the structure of the transgenic locus. Successful transgene expression depends to a large extent on the transgene being accepted as a genuine locus and not branded as a foreign sequence. Of particular importance, the production of dsRNA can trigger a potent and long-lasting posttranscriptional silencing mechanism that can affect both transgenes and homologous endogenous genes.