[962 |. 1). W A I SON 



specific template nucleotide sequence must release the finished protein. The 

 now vacant ribosome then becomes competent to receive the free end of 

 another (or perhaps even the same) messenger molecule and start a new 

 cycle ot protein synthesis. 



The realization that a single messenger molecule attaches to many ribo- 

 SOmes resolves a bothersome paradox which accompanied the messenger 

 hypothesis. About 2-4% of E. coli RNA is messenger 40 - 53 . Its average sed- 

 imentation constant ot 14s 54 suggests an average molecular weight about 

 500,000. This value may be too low since it is very difficult to completely 

 prevent all enzymatic degradation. There thus must be at least 6-8 70s ribo- 

 somes for every messenger molecule. It was very difficult to believe that only 

 10-20% of the ribosomes functioned at a given moment. For, under a 

 variety of conditions, the rate of protein synthesis is proportional to ribo- 

 some concentration 55 . Instead, it seems much more likely that, in vivo, almost 

 all ribosomes are active. During the preparation of cell extracts, however, 

 many ribosomes may lose their messenger and become inactive. If true, we 

 may expect that use of more gentle techniques to break open E. coli cells will 

 reveal larger fractions of fast-scdimenting active material. Already there are 

 reports 56 that over 50% of mammalian reticulocyte ribosomes exist as ag- 

 gregates of 5-6 80s particles. Furthermore, it is these aggregated ribosomes 

 which make protein, both /'// vivo and in vitro. 



Template Lifetime 



Under the above scheme a messenger molecule might function indefinitely. 

 On the contrary, however, the unstable bacterial templates function on the 

 average only 10-20 times. This fact comes from experiments done in Levin- 

 thal's laboratory 57 where new messenger synthesis was blocked by addition 

 of the antibiotic antinomycin D. Preexisting messenger (Bacillus subtilus 

 growing with a 60 minute generation time) then broke down with a half- 

 life of 2 minutes. Correspondingly, protein synthesis ceased at the expected 

 rate. A mcchanism(s) must thus exist to specifically degrade messenger mol- 

 ecules. Several enzymes (polynucleotide phosphorylasc and a K+ dependent 

 diesterase) which rapidly degrade free messenger are active in bacterial cell 

 extracts 58 . They function, however, much less efficiently when the messenger 

 is attached to ribosomes 59 . Conceivably, a random choice exists whether the 

 free forward-moving end of a messenger tape attaches to a vacant ribosome, 



s-I30 



