9 



Ultracentrifugal Studies of Microsomes from 



Starving, Nonproliferating, and 



Proliferating Yeast 



JAMES K. ASHIKAWA 



Dontier Laboratory of Biophysics and Medical Physics 

 University of California, Berkeley 



In the decade since Claude's successful isolation of microsomes from liver 

 and other tissue homogenates by differential centrifugation [1, 2], considerable 

 progress has been made toward elucidating their biochemical and morphological 

 characteristics. The microsomal "ribonucleoprotein" particles isolated from bac- 

 terial cells and from plant and animal tissues are rich in ribonucleic acid [3-10]. 

 In electron-microscopic observations these microsomal particles, either isolated or 

 in intact plant and animal cells, appear as spherical particles with diameters 

 ranging from 100 to 400 A. They occur either bound to the endoplasmic reticu- 

 lum or freely dispersed in the cytoplasmic matrix [7-14]. 



There are interesting studies indicating that microsomes may be actively in- 

 volved in protein and lipid synthesis [5, 15, 16]. In vivo [17-20] and in vitro 

 studies [15, 20, 21] have shown that labeled amino acids are preferentially in- 

 corporated into the microsomal proteins, suggesting synthetic activity. 



Since these microsome particles appear to be functional organelles in both 

 plant and animal cells, it is highly probable that their physicochemical proper- 

 ties will be altered by varying the physiological state of the cell. Preliminary 

 studies of microsomes isolated from starving, nonproliferating, and proliferat- 

 ing yeast cells have shown that several new ribonucleoprotein particles appear 

 during cell division [22, 23]. This paper presents further evidence thereon. 



EXPERIMENTAL PROCEDURE AND RESULTS 



Haploid yeast cells (Saccharomyces cerevisiae strain S. C. 7) aerobically cul- 

 tured for 24 hours at 30° C in yeast extract and dextrose ( YED 1 : 2 per cent) 



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