Proteins are synthesized by the ribosome and must generally fold to become

functionally active. Nascent protein folding can begin before synthesis is complete, but it

has been very difficult to study the mechanisms and consequences of folding on the

ribosome. We have developed an experimental system utilizing optical tweezers to follow

nascent protein folding, which allows us to observe the folding of single nascent protein

molecules in real time. We find that the ribosome slows the formation of stable tertiary

interactions and decelerates both folding and misfolding of T4 lysozyme, the model

protein used in our initial studies. Our results demonstrate that the ribosome not only

decodes the genetic information and synthesizes polypeptides, but also promotes efficient

de novo protein folding to the native state. Interestingly, we also found that nascent

protein folding can, in turn, regulate ribosome activity. Interactions between the nascent

chain and the polypeptide exit tunnel in the ribosome can cause decreased elongation

rates and even arrest. We show that nascent chain folding in the vicinity of the ribosomal

tunnel exit releases arrest by generating mechanical force. Our results provide

mechanistic insight into how nascent chain folding and elongation are coupled during

protein synthesis by the ribosome.