RNA AND PROTEIN SYNTHESIS IN DEVELOPING 

 SEA URCHIN EGGS 



Paul R. Gross 



Biology Department, Massachusetts 

 Institute of Technology, Cambridge, Massachusetts 



I propose to summarize here what I believe 

 are some important points emerging from the 

 recent study of biochemical events, especially 

 those involved with macromolecule synthesis, 

 that follow immediately after fertilization of 

 sea urchin eggs (1). There appears to be a 

 necessity for the existence of systems control- 

 ling protein synthesis at the level of translation 

 of RNA messages (2). Experiments on the 

 early course of development are now no longer 

 unique in demonstrating the existence of trans- 

 lation control. However, in fact, these were 

 among the first in which the necessity for such 

 a conclusion appeared. There are a number of 

 other kinds of developing and differentiating 

 systems in which evidence of control at this 

 level is available. Someof these will undoubtedly 

 be considered as these discussions progress. 



The observations that led to the postulation 

 of translation control follow. Synthesis of pro- 

 teins, which is an inevitable accompaniment of 

 early development, may be uncoupled from the 

 synthesis of new RNA (e.g., 3). This uncoupling 

 can be absolute and may last for a very long 

 time. When the observation was first made, it 

 was surprising because the situat'.on with re- 

 spect to messenger function of RNA in micro- 

 bial cells would not necessarily have led to 

 the prediction of such a level of control, since 

 in microbes the continuation of protein synthesis 

 requires concomitant synthesis of RNA mes- 

 sages whose half life is short, relative to the 

 length of the cell cycle. In a system allowing 

 the synthesis of protein to go on in the absence 

 of new synthesis of messenger RNA, it must be 

 true that either such synthesis doesn't require 

 messages, or that the messages are very stable. 



The possibility that protein synthesis ac- 

 companying early development may not require 

 messenger RNA could be established in a num- 



ber of ways. One could, for example, look for 

 polyribosomes in embryos. One could make 

 estimates of the fraction of the early synthesis 

 that occurs on polyribosomes, and if most of 

 the synthesis does occur there, then it is rea- 

 sonable to assume that protein synthesis does 

 require messenger RNA and associated ribo- 

 somes. Such seems to be the case (4, 5, 6). There 

 is no primary site that we have been able to 

 detect for protein synthesis in sea urchin eggs 

 other than ribosomes associated with a length 

 of highly nuclease-sensitive RNA. The extent 

 to which those objects, themselves, are asso- 

 ciated with other, perhaps larger, structures 

 is an interesting point that I hope will come up 

 in the discussion later. At least, the poly- 

 ribosome, itself, is the unit on which early 

 proteins are made. Since under uncoupling 

 conditions new messages are not made, old 

 ones must supply the information for transla- 

 tion. That much alone suggests that these mes- 

 sages must be stable. Although at the time the 

 observations were made, that was in itself a 

 moderately radical proposal, the existence of 

 very stable messages has since been shown in 

 several cases (e.g., 7). Stable messages seem 

 now to be not at all exceptional in higher cells, 

 even in relation to the long inter-mitotic time. 

 Our starting point was the independence of 

 new protein synthesis from new RNA synthesis 

 in embryos; that is, messages directing the 

 early synthesis must have been present in the 

 egg before it was fertilized. There is, quite 

 generally, a long period between the time that 

 an egg is completed and set aside in a condition 

 of relative dormancy in the ovary, and the time 

 it is released from the mother to be fertilized. 

 Hence, the further suggestion that the templates 

 for early embryonic protein synthesis are not 

 only very stable in use, but may be stored for 



