Final answer:
The factor responsible for releasing the now-uncharged tRNA during the termination phase of protein synthesis is Release Factor (RF), specifically the RF3, which is associated with the presence of a termination codon.
Step-by-step explanation:
The factor that triggers the release of the now-uncharged tRNA is Release Factor (RF), specifically RF3 in this context. During the termination phase of protein synthesis, the release factors play critical roles. Once a termination codon is present in the A-site of the ribosome, RF1 and RF2 help in the hydrolysis of the peptidyl-tRNA bond. This hydrolysis leads to the release of the free polypeptide. Subsequently, RF3, which is associated with GTP, aids in the detachment of the last tRNA from the P-site and contributes to the dissociation of the ribosomal subunits. In contrast, EF-G, also known as translocase, is involved in the translocation phase of elongation, not termination.