2.12 Turning Points 557 Am

essay turning points Wee beasties How a tiny observation turned out to have a controlling influence.

before they had grown to their Paul Nurse normal size. major turning point in my I called over my friend and scientific life involved not colleague,Pierre Thuriaux,who Aonly a group of individuals was visiting from Bern. He was (important as they were for what aware of my limited genetic happened), but also a micro- skills and gently suggested that organism — or to be more precise, my plates were contaminated a mutant microorganism. The with another microorganism. time was 1974 and the place was It was another day before we Murdoch Mitchison’s laboratory and Mitchison were convinced in the zoology department at the that it was really fission yeast, University of Edinburgh. Mitchison albeit a small one. During our had been interested in the cell- conversations we recalled a division cycle since the 1950s, seminar from a colleague in the working mainly with the fission department of genetics, Henry yeast Schizosaccharomyces pombe. Kacser, on the control of meta- He recognized that the length bolic flux. Kacser had argued of this rod-shaped, single-celled that although control was organism was a good marker for The author (left) and Pierre Thuriaux in more relaxed settings, 1974. distributed through all steps of how far the cell cycle had pro- Below, the ‘wee’ mutant (panel b) in comparison to other yeast cells. a metabolic pathway, some gressed, which made fission yeast steps could be more rate- a convenient model for cell cycle studies. It limiting than others.Was it possible that this was this microorganism which transformed small-sized mutant (soon to be called ‘wee’to my scientific life. reflect its Scottish origin) had a defective The previous year, I had been a graduate gene for a major rate-limiting step in the student in Norwich studying amino-acid pathways controlling cell-cycle progression? pools in fungi. This project required me to This simple interpretation turned out to be nurture a rather temperamental Beckman correct when it was eventually demonstrated amino-acid analyser, and I often found that Wee1 regulates the cyclin-dependent myself keeping this machine company kinase that acts as the major engine of the throughout the night — just to make sure it fission-yeast cell cycle. kept running! During these long night vigils, Next, these mutants were screened visually What lessons did I learn from this turning I read numerous research papers to keep me using a microscope to find colonies that con- point in my career? The first was the impor- awake and two from Lee Hartwell attracted tained elongated cells, which indicated that tance in biology of letting the organism you my attention. These described the isolation they could not complete their cell cycle and are studying guide you in the right direction of temperature-sensitive cell-division-cycle divide, although they continued to grow. By to solve your problem. By using microscopic (cdc) mutants in budding yeast (Saccharo- the end of the year I had only found approxi- visual inspection, I was not being too myces cerevisiae), which failed to progress mately 30 cdc mutants. restricted in my search for mutants. If some- through the cell cycle when grown at a high I devised an enrichment procedure to thing unexpected turned up, I could see it. temperature. Immediately, I decided that I try and speed things up. I centrifuged cell This allowed mutants that no one had imag- wanted to do something similar and, being populations through a density gradient to ined existing before to be observed and familiar with Mitchison’s work, thought separate large, elongated cells from normal- thought about, opening up new avenues of fission yeast would be a good organism for sized cells. The larger cells, some of which enquiry. Serendipidity was given a chance to this project. I contacted Mitchison, who was would be cdc mutants, should make it to play a role, allowing nature to constrain and enthusiastic about me coming to Edinburgh, the bottom of the gradient, and these frac- guide the infinite possibilities that occur to but wisely suggested that I should first tions were plated out. After isolating a few the human mind. learn and become familiar with genetic mutants this way, something strange A second lesson was the importance of techniques.So I went to Urs Leupold in Bern, happened. There was a clump of cells in the spending time trying to understand the real where he and his colleague, Peter Munz, middle of the microscope field that seemed biological significance of observations made taught me fission-yeast genetics. I finally to be smaller than normal. My first response on cells and organisms before undertaking arrived in Edinburgh at the start of 1974, was disbelief — how could such small cells detailed investigations of the molecular with my newly acquired (but rather limited) have got to the bottom of the gradient? This mechanisms involved. Understanding the genetic knowledge, and started isolating cdc thought was slowly replaced by another one biology properly is as important as the mutants in fission yeast. — what was the significance of these tiny subsequent mechanistic insight. However, the isolation procedure was cells? Fission yeast cells usually divide at a One final thought — why were the wee very laborious and so progress was very slow. constant cell length, so if these cells were mutant cells at the bottom of the gradient? I First, temperature-sensitive mutants (those shorter than normal, this must mean that don’t know, perhaps it was because they unable to form colonies at a high temperature) they had divided at a small size.This was only clump easily or maybe I was just very lucky.■ had to be identified by replica plating yeast possible if cells had progressed more rapidly Paul Nurse is in the Rockefeller University, colonies from a low to a high temperature. through their cycle, dividing prematurely 1230 York Avenue, New York 10021, USA.