88. An in vitro translation system containing microsomes was used to translate an mRNA encoding a secretory
protein lacking the stop codon. Which one of the following outcomes can be expected?
1. The protein will not be translated
2. The protein translation will start but stop after some time
3. The protein will be fully synthesized but not incorporated into the microsomal membrane
4. The protein will be synthesized and incorporated into the microsomes but will not be released from the
ribosomes

Introduction: Understanding In Vitro Translation with Microsomes

In vitro translation systems are powerful tools used to study protein synthesis and the incorporation of proteins into cellular components such as microsomes. Microsomes are vesicles derived from the endoplasmic reticulum (ER) and are crucial in studying the synthesis of secretory proteins, which are typically translated in association with the ER membrane. However, the presence of certain genetic anomalies, such as the absence of a stop codon, can affect the translation and the protein’s behavior.

In this case, we are considering a secretory protein mRNA that lacks a stop codon. Let’s explore the potential outcomes of this translation process in a system containing microsomes.

Mechanism of Protein Synthesis and the Role of the Stop Codon

1. Translation Process: Translation begins when ribosomes bind to the mRNA and synthesize the protein. During translation, ribosomes read the mRNA codons, incorporating the corresponding amino acids into the growing polypeptide chain.

2. Stop Codon: The stop codon signals the end of translation, instructing the ribosome to release the newly synthesized protein. Without a stop codon, translation would continue indefinitely.

Expected Outcomes

Now, let’s examine the expected outcomes when an mRNA encoding a secretory protein lacks a stop codon in an in vitro system containing microsomes:

1. The protein will not be translated:

• This is unlikely because the absence of a stop codon does not prevent translation from starting. Translation would initiate normally, but without the stop codon, the process cannot be completed.

2. The protein translation will start but stop after some time:

• This is also incorrect because without a stop codon, the ribosomes will not know when to stop translating the protein. The protein translation will continue indefinitely, leading to an elongation of the polypeptide chain without termination.

3. The protein will be fully synthesized but not incorporated into the microsomal membrane:

• This outcome is also unlikely. While translation might continue indefinitely, the absence of a stop codon does not directly prevent the incorporation of the protein into the microsomes. The protein would likely be synthesized, but the lack of termination could lead to difficulties in processing and translocation into the microsomal membrane.

4. The protein will be synthesized and incorporated into the microsomes but will not be released from the ribosomes:

• This is the most likely outcome. Without a stop codon, translation will continue indefinitely, causing the protein to be synthesized and incorporated into the microsomal membrane. However, because translation does not terminate, the ribosome will not detach from the mRNA, preventing the normal release of the protein. This would likely result in the protein being stuck at the ribosome-microsome interface, unable to be fully processed or released.

Conclusion

The correct outcome when translating an mRNA for a secretory protein that lacks a stop codon is:

Correct Answer: The protein will be synthesized and incorporated into the microsomes but will not be released from the ribosomes.

This article explains the consequences of lacking a stop codon in a secretory protein mRNA in an in vitro translation system with microsomes. The expected result is that the protein will be synthesized and incorporated into the microsomal membrane, but not released from the ribosomes due to the absence of translation termination.