Correct option is C
Explanation-
The free r-protein(s) bind to the mRNA(s) and downregulate their translation. This is the established mechanism in E. coli.
Each r-protein (or a group of them) is often encoded in operons—single mRNA transcripts with multiple r-protein genes. The mRNA has a regulatory sequence in the 5' UTR or nearby regions.
This sequence often mimics the structure of rRNA that the r-protein normally binds during ribosome assembly. When there is excess free r-protein (unbound to rRNA), it binds to this mRNA regulatory structure.
This binding blocks ribosome binding sites or distorts mRNA structure, making it inaccessible for translation initiation.
Incorrect options-
Option a - The free r-protein(s) often bind to corresponding DNA sequence and activate transcription of rRNA genes to increase rRNA availability.
r-proteins do not regulate transcription of rRNA genes. They regulate their own translation, not rRNA synthesis.
Option b - The free r-protein(s) bind to RNA polymerase and repress transcription of the r-protein genes to decrease the availability of their mRNAs.
They regulate at the translational level, not by interfering with RNA polymerase or transcription.
Option d - The free r-protein(s) bind free NTPs which then activates their cryptic ribonuclease activity leading to the degradation of their mRNAs.
This is not how translational feedback works for r-proteins. They do not degrade mRNA via ribonuclease activation.
So, the correct answer is option c - The free r-protein(s) bind to the mRNA(s) and downregulate their translation.
