After receiving his Ph.D., Altman embarked upon the first of two research fellowships. He joined Matthew Meselson's laboratory at Harvard University to study a DNA endonuclease involved in the replication and recombination of T4 DNA. Later, at the MRC Laboratory of Molecular Biology in Cambridge, England, Altman started the work that led to the discovery of RNase P and the enzymatic properties of the RNA subunit of that ribozyme. John D. Smith, as well as several postdoctoral colleagues, provided Altman with very good advice that enabled him to test his ideas. "The discovery of the first radiochemically pure precursor to a tRNA molecule enabled me to get a job as an assistant professor at Yale University in 1971, a difficult time to get any job at all".