66 MICROSOMAL PARTICLES 



THE 40 S COMPONENT 



Variations in the concentration of the 40 S component due to cultural changes 

 were far greater than for 13 S. Cells grown in peptone contained higher con- 

 centrations of the 40 S component than those at a corresponding phase of 

 growth in mineral salts media, and for both types of medium the concentration 

 increased during cell division and decreased in the stationary phase. Since the 

 40 S component carries most of the RNA of E. coli (Schachman et al. [1]), these 

 observations agree with Wade and Morgan [18], who found a higher RNA 

 content for dividing than for resting cells; but we have never examined extracts 

 of cells harvested from stationary-phase cultures that were entirely devoid of 

 40, 29, or 20 S components although we adopted the same extraction procedures 

 as these authors in several experiments. Cells that had remained 2 hours in a 

 stationary culture were not distinguishable from those late in logarithmic 

 growth; and a significant reduction in the 40 S component could only be seen 

 after 8 hours. 



The most striking effect observed was the disappearance of the 40 S boundary 

 when cells were resuspended in phosphate buffer of the same strength (0.13 M) 

 as that in the growth medium from which they were harvested. Apparently 

 no permanent damage is done to the cells by this treatment because, on addi- 

 tion to the buffer of glucose, Mg ++ and (NH4) 2SO4 or peptone, growth occurs 

 with little or no lag period. These observations are illustrated in figure 3 with 

 patterns of extracts from cells grown to the stationary phase in mineral salts 

 medium (a), which were then transferred to peptone medium and examined 

 in mid-logarithmic phase (b) and at the end of this phase (c). The augmen- 

 tation of 40 S during cell division in peptone is evident; and, in (c), 29 S has 

 also increased. Cells that gave pattern (c) were incubated for 30 minutes in 

 0.13 M phosphate, and most of the 40 S disappeared to leave two small peaks 

 of 43 and 40 S (d) ; on further incubation only traces of material remained 

 that sedimented in this region (e). Peptone and glucose were now added to 

 the suspended cells; growth resumed, and (/) gives the pattern after 1 hour. 



Some of the conditions for stabilizing the 40 S component within whole cells 

 were investigated. Dr. A. Tissieres informed us that its disappearance de- 

 pended on the concentration of phosphate in the buffer used to suspend the 

 cells, and we have found a considerable diminution of the rate of loss of 40 S 

 when the phosphate is reduced from 0.13 M to 0.066 M. It does not appear that 

 the effect of orthophosphate is specific, however, since disappearance of the 

 40 S component has been shown for whole cells resuspended in potassium 

 chloride of the same ionic strength as 0.13 M phosphate. The importance of 

 magnesium ions is shown by the results of figure 4. If the growth medium is 

 complete except for addition of Mg ++ , 40 S is lost (fig. 4£). On addition of 

 Mg ++ , 40 S is stabilized and cell division may begin; but Mg ++ may stabilize this 

 component even in cells that are not able to divide because the source of energy 

 has been omitted (fig. 4<r). Wade and Morgan [18] have shown an association 



