/¡Ç' is now exposed and translation is activated
mRNA is normally made using the non-coding strand of DNA as a template. Such mRNA is also known as sense RNA. If RNA is made using the coding strand as a template, it will be complementary in sequence to m RNA and is known as anti-sense RNA. The sense and antisense strands of RNA can base pair.
Non-coding strand 3' Coding strand 5'
Transcription ds RNA
it from binding to the ribosome and being translated (Fig. 11.09). In practice, antisense RNA is not made by transcribing the non-template strand of the same gene that gives the mRNA. Another, quite distinct "anti-gene" is used for making the antisense RNA.
Bacterioferritin is the protein used by bacteria to store surplus iron atoms. The bfr gene encodes bacterioferritin itself and the anti-bf gene encodes the antisense RNA. Since only a relatively short piece of antisense RNA is needed to block the mRNA,
FIGURE 11.09 Antisense RNA Regulates Bacterioferritin Synthesis
The bacterial chromosome contains genes for both bfr m RNA and anti-bfr RNA. If both RNA molecules are transcribed the anti-bfr RNA pairs with the bfr mRNA and prevents it from being translated. When iron is plentiful the anti-bfr gene is not expressed and only the bfr mRNA is produced. Under these conditions translation of the bfr mRNA to give bacterioferritin can take place.
the anti-gene is similar in sequence but shorter than the original gene. When the iron concentration in the culture medium is low, bacterioferritin is not needed, but it is made if the iron level goes up. The bfr gene itself is transcribed to give mRNA in both conditions. However, the anti-bfr gene is controlled by a regulatory protein known as Fur (Ferric Uptake Regulator), which senses iron levels.
When plenty of iron is present, Fur acts as a repressor and turns off the transcription of a dozen or more operons needed for adapting the cell to iron scarcity. These include genes for several iron uptake systems designed to capture trace levels of this essential nutrient. In addition, Fur plus iron turns off the anti-bfr gene, which turns on the production of bacterioferritin (Fig. 11.09). In low iron the anti-bfr gene is transcribed to give antisense RNA. This prevents synthesis of the bacterioferritin protein when iron is scarce. Thus, by using antisense RNA, one gene can be regulated the opposite way to a group of others although all respond to the same stimulus.
Artificially synthesized antisense RNA will interfere with gene expression or any other cell process involving RNA. For example, antisense RNA is being tested experimentally to suppress cancer by stopping chromosome division.
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