■104 MAHLON B. HOAGLAND 



comes indistinguishable from particle RNA in sedimentation properties. 

 Thus, it must become bound to microsomal RNA in some reasonably firm 

 way. Further, it was clear that the amino acid also became associated with 

 particle RNA, and the kinetics of this reaction were similar to the kinetics 

 of attachment of the transfer RNA to particles. 



These observations are consistent with the adaptor hypothesis in most 

 respects. We should expect to find that amino acid bound to soluble adap- 

 tor would appear at least in a transitory way on particle RNA in company 

 with the adaptor. This intermediate state would be the template or particle 

 RNA hydrogen bonded, by complementary base pairing, to the adaptor 

 RNA, the amino acid still being attached to the latter. This stage would be 

 transitory, for the amino acid would condense with its neighbors, be re- 

 moved from the adaptor, and the latter would then be expected to return 

 to the soluble milieu, thus completing a catalytic cycle. It should be said, 

 however, that although the association of amino acid and transfer RNA (or 

 part of it) with the particle RNA appears to be transitory and in a steady 

 state situation, this has not yet been proved. 



We have observed that there is apparently little species specificity in the 

 reactions we have been discussing. Particles from one animal source are 

 able to accept amino acids from transfer RNA derived from other species. 

 This is consistent with the theory. We should predict, however, that the 

 proteins synthesized would be those characteristic of the organism from 

 which the particles had been derived, and this should soon be experimentally 

 attackable. Indeed, Schweet has already shown that soluble enzymes from 

 guinea pig assist rabbit reticulocyte ribosomes in the synthesis of hemo- 

 globin. 108 Brachet 183 found that protein synthesis in whole onion root tips 

 depleted of their sRNA by ribonuclease could be restored by adding back 

 RNA from either yeast or onion root. However, there is, as we have men- 

 tioned, evidence 177 of species differences in the extent to which activating 

 enzymes can attach amino acids to a given sample of sRNA. Thus the 

 picture is still far from clear. 



If the hypothesis is correct, we should expect that at least part of the 

 particle RNA would be free to react with the adaptor, i.e., its hydrogen 

 bonds should be unoccupied. As we have mentioned, Doty and his associ- 

 ates have obtained tentative evidence that a large fraction of the particle 

 RNA is not involved in internal hydrogen bonding and could thus be avail- 

 able for reactions with adaptors. 



Ts'o and Lubell 232 have made a calculation based on data from rabbit 

 reticulocytes, to determine whether the frequency of collisions of transfer 

 RNA molecules with ribosomes would be high enough to account for the 

 rate at which these cells synthesize hemoglobin. They estimate that a vol- 



232 P. O. P. Ts'oandA. R. Lubell, Abstr. Am. Chcm. Soc. ,135th Meeting, Boston (1959). 



