290 H. H. ROBERTS. K. J. URITTEN, AM) U. J. MCCARTHY 



\\'li('n tlicsr conH'ctions arc made the rihosonics of A', coli show three 

 prominent groiii);> at 30.6 di IS. oO.O d= IS, and 69.1 ± IS. These are 

 usually roinided off to 30, 50, and 7()S for identification (Tissiei-cs rt (iL, 

 1 909 1 . 



Hihosonios extracted from E. coli at 10- A/ Mg show predominantly 

 the 70S (or larger) form (Fig. 11, but when the extraction is carried out 

 in 10 ' Mg or when the Mg concentration is reduced to 10 ' M after 

 extraction th(> large particles give rise to equal numbers of 30S and 50S 

 (Fig. 2). This fact, together with electron microscope pictures which 

 show shapes consistent with a 3O-50S complex (Fig. 3), (Hall and 

 Slayter, 1959; Huxley and Zubay, 1960) indicates that the 30S and 50S 

 ribosomes associate reversibly to form 70S particles. 



In addition, ribosomes of sedimentation coefficients greater than 70S 

 are observed. One of these (al)out lOOS) appears to be a dimer of the 

 70S particle (Fig. 3). The reactions are not simi)le, how(>ver. An inter- 

 mediate group (called 85S for identification) also appears and its 

 sedimentation coefficient as well as its proportions depends on the mag- 

 nesium concentration (Britten and McCarthy, 1959). As this 85S grouji 

 yields 30S and 50S ])articles in equal numbers when the magnesium 

 concentration is reduced, it appears to be due either to an altered shape 

 or hydration of the 70S or lOOS particles or to a rapid equilibration 

 between the 70S and lOOS forms which yields an interme<;liate sedimenta- 

 tion coefficient. 



C. OTHER INFLUENCE.S ON RIBOSOME P.\TTERNS 



The sedimentation pattern also depends upon the metabolic state of 

 the cell at the time of breaking and upon the method of breaking the 

 cells (Fig. 4). McCarthy has shown that cells rapidly accumulate the 

 lOOS particles when their energy supply (glucose) is exhausted and 

 rev(>rt with equal rapidity to the normal pattern when glucose is restored 

 (McCarthy, 1960). Similar changes also appear when mutants are 

 deprived of a refjuired metabolite. There is little difference, however, in 

 ribosomes taken from different phases of the growtli cycle in synchronized 

 cultures (Britten et nl, 1960). 



Prolonged incubation of growing cells in ]\Ig-free media reduces the 

 ribosome content to less than 5%. The ribosome content is restored (fol- 

 lowing an exponential growth curve) when the magnesium of the 

 medium is replenished (McCarthy, 1962). 



The ribosome pattern does not result simply from an equilibrium 

 among two basic units and combination thereof. Some 30S and 50S 

 particles are present in the cell juice even when the magnesium concen- 

 tration is high. These are sometimes designated "native" 308 and 50S 



