Transcription regulation lets an organism “pull out” from its genome what it needs when it needs it and “put it back” until it’s needed again. However, some organisms prefer to “keep” everything in front of them and use it on the spot. They essentially transcribe everything at once, with some differences that we’ll discuss later, and leave regulation for the later stages—a process known as polycistronic transcription. This is common in prokaryotes and rare in eukaryotes.
Monocistronic vs. Polycistronic Transcription
Polycistronic transcription, where a single mRNA molecule encodes multiple proteins, is a well-characterized phenomenon in prokaryotes, particularly bacteria and archaea. This mechanism is closely tied to the operon model, where multiple genes with related functions are transcribed together into a single mRNA and translated into proteins. In contrast, eukaryotes are generally thought to follow a monocistronic model, where each mRNA encodes a single protein. However, recent research has shown that polycistronic transcription is not exclusive to prokaryotes. Some eukaryotes, especially certain protists, also utilize this form of gene expression, albeit through different mechanisms.
Polycistronic Transcription in Eukaryotes: A Focus on Protists
In the operon model of prokaryotes, functionally related genes are organized in one operon so that the entire pathway is transcribed together when needed. However, in protists such as Trypanosoma brucei, functionally unrelated genes are clustered into several large polycistronic transcriptional units, separated by strand-switch regions. Transcription initiation occurs at these strand-switch regions, where bi-directional transcription takes place. Remarkably, no promoter regions have been identified, and initiation relies on histone variants and modifications.
In fact, only one promoter is recognized by RNA polymerase II, and it is for a special transcript called the splice leader (SL). In the nucleus, the SL gets capped and is attached by trans-splicing. This is how protists manage their polycistronic mRNAs. They perform a process called trans-splicing, in which the SL, along with the spliceosome, splices the polycistronic mRNAs into monocistronic ones that are then polyadenylated.
To regulate gene expression, protists primarily rely on post-transcriptional mechanisms, including processes like RNA modification and editing.
Do Eukaryotes Pay for Polycistronic Transcription?
Polycistronic transcription in protists represents a trade-off between efficiency and complexity. While it can streamline the expression of related genes and reduce the need for multiple transcription events, it also introduces additional challenges in processing and regulation. Unicellular organisms can avoid the demanding process of transcription initiation but face the cost of post-transcriptional processing and regulation.
In many cases, protists have evolved specialized mechanisms to manage the challenges of polycistronic transcription, suggesting that the benefits outweigh the potential costs. However, disruptions to this system could lead to negative consequences, highlighting the delicate balance that must be maintained for polycistronic transcription to be advantageous.
A Safety Net
Unicellular protists exhibit a surprising level of complexity despite their single-celled nature. They often inhabit environments that are highly variable and can change rapidly, requiring them to adapt quickly to survive. Rapid translation control allows them to adjust their protein synthesis in response to environmental changes and evade the immune system of the host. Additionally, protists often have complex life cycles with different stages, needing to switch the translation of specific proteins for each stage based on host interaction and environmental factors. Thus, the bulk of transcripts can act as a safety net, allowing them to respond quickly to environmental emergencies.
Although polycistronic transcription is not restricted to protists among eukaryotes—having also been identified in C. elegans and Drosophila melanogaster—it seems not to be the preferred mechanism for most eukaryotes.
References:
Teixeira, S. M., de Paiva, R. M. C., Kangussu-Marcolino, M. M. & DaRocha, W. D. Trypanosomatid comparative genomics: Contributions to the study of parasite biology and different parasitic diseases. Genet Mol Biol 35, 1–17 (2012).
Lukeš, J., Leander, B. S. & Keeling, P. J. Cascades of Convergent Evolution: The Corresponding Evolutionary Histories of Euglenozoans and Dinoflagellates. Proceedings of the National Academy of Sciences of the United States of America 106, 9963–9970 (2009).
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