Final answer:
The yeast Ty virus-like particle is missing the ENV protein, which in retroviruses forms the envelope that encapsulates viral RNA. Instead, the Ty VLP uses a Gag protein for encapsulation and does not form infectious particles like retroviruses, but facilitates retrotransposition within the yeast cell.
Step-by-step explanation:
The question relates to the yeast Ty virus-like particle (VLP), which is part of the yeast's retrotransposition mechanics. Specifically, it refers to what is "missing" from these structures. When we analyze the retrotransposons and the process of retrotransposition, we see that the mechanism closely mirrors that used by retroviruses. However, there's a crucial element missing in yeast which is normally found in retroviruses: the ENV protein. This protein, encoded by the ENV gene, is responsible for forming the envelope which encapsulates the retroviral RNA when producing infectious virus particles.
In the case of the yeast Ty element, instead of an ENV protein, the structural protein Gag makes up VLP. The VLP encapsulates retrotransposon RNA and the accompanying enzymes in the cytoplasm. Unlike retroviruses, VLP-coated retrotransposons do not lyse the host cell to release infectious particles. The VLP simply loses its coat, re-enters the nucleus, and integrates into the genomic DNA. Therefore, one can conclude that the yeast Ty VLP is missing the viral ENV protein, stopping it from forming infectious viral particles akin to retroviruses.