While a postdoctoral fellow working with Aaron Klug and Francis Crick at the MRC in the 1970s, Kornberg discovered the nucleosome as the basic protein complex packaging chromosomal DNA in the nucleus of eukaryotic cells. Chromosomal DNA is often termed "Chromatin" when it is bound to proteins in this manner, reflecting Walther Flemming's discovery that certain structures within the cell nucleus would absorb dyes and become visible under a microscope.

Within the nucleosome Kornberg found that roughly 200 bp of DNA are wrapped around an octamer of histone proteins. With Yahli Lorch, Kornberg showed that a nucleosome on a promoter prevents the initiation of transcription, leading to the recognition of a functional role for the nucleosome, which serves as a general gene repressor. Order of authorship in the original publication: Lorsch, Lapointe, Kornberg.

Kornberg discovered that transmission of gene regulatory signals to the RNA polymerase machinery is accomplished by an additional protein complex dubbed the Mediator. As noted by the Nobel Prize committee, "the great complexity of eukaryotic organisms is actually enabled by the fine interplay between tissue-specific substances, enhancers in the DNA and Mediator. The discovery of Mediator is therefore a true milestone in the understanding of the transcription process."

Kornberg devoted two decades to the development of methods to visualize the atomic structure of RNA polymerase and its associated protein components. Initially, Kornberg took advantage of expertise with lipid membranes gained from his graduate studies to devise a technique for the formation of two-dimensional protein crystals on lipid bilayers. These 2D crystals could then be analyzed using electron microscopy to derive low-resolution images of the protein's structure. Eventually, Kornberg was able to use X-ray crystallography to solve the 3-dimensional structure of RNA polymerase at atomic resolution. Through these studies, Kornberg created an actual picture of how transcription works at a molecular level. According to the Nobel Prize committee, "the truly revolutionary aspect of the picture Kornberg has created is that it captures the process of transcription in full flow. What we see is an RNA-strand being constructed, and hence the exact positions of the DNA, polymerase and RNA during this process.” With P Cramer , D A Bushnell, J Fu, A L Gnatt, B Maier-Davis, N E Thompson, R R Burgess, A M Edwards, P R David. Digital facsimile from PubMedCentral at this link.

In 2006 Kornberg was awarded the Nobel Prize in Chemistry "for his studies of the molecular basis of eukaryotic transcription."