Qril 14, lcfG0 Dr. Jacq.~?s Monod Institut Pasteur 28 Rue du Dr. R~LZX Paris 15, France Deer Jacr;uee, The arrival of And& has brought us first-hand newa of what goes on in Paris and I wistfully remetier the pleasent weeks spent last year in your laboratory. I hope to visit you for a. few days in June, after n meeting in Louvnin. Will you be in Paris al.1 the month of June? Meanwhile, I would like to have some information and some strafns, which will help with the Pl lac problem. 1, The enzyme story has not adv`anced very far. We have now good evidence that the early aceelerP;tion in enzyme rate formation Is real ad not due to chaages in medium, washing of cells, etc. We know that UV doses that prevent phage multiplication only reduce very slightly the rate of enqme formation; we are using this method to measure the relative W cross sections of phsge, z, and i regions. The effect of immuni+;y on enzyme synthesis is more complex than It seemed to be and, as in the case of galactokinase, It probably reflects interactions between repressors. We zre preparing a modified Benzer experiment, to decide how many cells make enzyme. If it does not work, the fluorescent substrate method will be needed. Have you perfected it, or should we go ahead and try? 2. We have obtained Pl lxc phsges that carry the isz'y4 set of genes and also some that appear to carry only the ys region. In order to stud\i the nature of the lac fragment, I need several mnpped z' and y' mutants." I have 2G!50, 2340, nnd 3.OOU from your collection. Could you send me some other ones? Could you include one strain that is i-y-z', and also one strain that is F", non-prototrophic, but with n lac' gene different from the one in F678 (which behaves strangely in some crosses)? 3. Strnin 3.OOl.J behaves strangely. It appears to be z- rather than y-. It maps between