Steitz completed postdoctoral research at the Medical Research Council (MRC) Laboratory of Molecular Biology (LMB) at the University of Cambridge (UK), where she collaborated with Francis Crick, Sydney Brenner, and Mark Bretscher. At the LMB, Steitz focused on the question of how bacteria know where to start the "reading frame" on mRNA. In the process, Steitz discovered the exact sequences on a mature RNA virus encoding three proteins where the virus mRNA binds bacterial ribosomes to produce proteins. In 1969 she published a seminal paper in "Nature" showing the nucleotide sequence of the binding start points.