On Joshua Lederberg's "Profiles in Science" NLM website, searching for "Esther Zimmer" returns the following PDF file. "E. Zimmer" is discussed as item "2)" in the third paragraph.
Click to view PDF file of Letter from E. L. Tatum to G. W. Beadle, June 7, 1946.
Note: the file opens in a new instance of your browser.
This specific piece of correspondence may be found on Joshua
Lederberg's NLM website, identified as with the ID:
bbapdr.
http://profiles.NLM.NIH.gov/BB/