A study has shown for the first time that the building blocks of proteins can be assembled without instructions from DNA or messenger RNA (mRNA).

A protein, Rqc2, was found playing a role similar to that of mRNA and specifying which amino acids, the building blocks of proteins, to be added in cell mechanism.

"In this case, we have a protein playing a role normally filled by mRNA," said Adam Frost, assistant professor at University of California, San Francisco.

"This surprising discovery reflects how incomplete our understanding of biology is," said first author Peter Shen, a postdoctoral fellow in biochemistry at the University of Utah in the US.

The researchers added that the findings have implications for new therapies to treat neurodegenerative diseases such as Alzheimer's, Amyotrophic lateral sclerosis (ALS) or Huntington's.

The researchers described that ribosomes are machines on a protein assembly line, linking together amino acids in an order specified by the genetic code.

When something goes wrong, the ribosome is generally disassembled, the blueprint is discarded and the partly made protein is recycled.

The new study, however, revealed that before the incomplete protein is recycled, Rqc2 can prompt the ribosomes to add just two amino acids (of a total of 20) - alanine and threonine - over and over, and in any order.

The nonsensical sequence likely serves specific purposes. The code could signal that the partial protein must be destroyed, or it could be part of a test to see whether the ribosome is working properly, the researchers noted.

For the study, they fine-tuned a technique called cryo-electron microscopy to flash freeze, and then visualse, the quality control machinery in cells in action.

The findings appeared in the journal Science.