May 16, 1947
Dear Tracy:
I found the enclosed check in one geological strata of the pile that occupies my desk and hasten to send it on to you before it disappears completely.  I hope you will pardon the delay.
I hear that marvelous things are happening in your laboratory, but that of course is not surprising.  We are still plugging along at transformations and are succeeding in transforming some tagged strains, but do not as yet have a suitable method of preparation and assay of the active material.  I have no doubt, from the experience that we have had so far with this problem, that it was this that was the basis of the melibiose experiments.  It will, however, take a lot of patient work to unravel the details and get a controllable and consistent experimental set-up.
We have been rather successful in other directions in trying to find out the factors controlling the appearance and disappearance of enzymes in the cytoplasm.  Thus, we have been able to get enzymes formed by the use of analogs rather than the substrates themselves, and also under conditions in which the adapting substrate could not be actually utilized.  This is providing us with a rather interesting tool for the further investigation of the problem.
We have also been able to uncover a hitherto unsuspected phosphate-containing protein which is extremely active during the synthesis of new proteins by the cell.  It cerainly raises the question of whether nucleic acid-containing proteins are the whole story.
I am extremely busy trying to get ready to leave for the summer and hope I see you soon.  Let me know what's up.
Sincerely yours,
S. Spiegelman