80 MICROBIAL CELL WALLS 



Pathways for Cell-Wall Biosynthesis 



The evidence that the uridine and cytidine nucleotides 

 containing a number of typical cell-wall compounds are 

 indeed cell-wall intermediates is most convincing when their 

 compositions are compared with cell walls. Yet the hard 

 fact remains that convincing incorporation or transfer of 

 the muramic-acid-peptide moiety of the nucleotide to the 

 wall has not been demonstrated. This, of course, has 

 prompted the sceptics to say "I told you so!" However, the 

 problem of getting such a nucleotide through the existing 

 wall into the right part of the membrane and close to ac- 

 ceptor sites on the wall must be a tremendous one. Stromin- 

 ger has also clearly pointed out that very low levels of 

 transfer w^ould not be surprising if one attempted to assess 

 the probable number of acceptor sites on the wall. "If it 

 is assumed that intact organisms contain of the order of 1000 

 acceptor sites per cell, then all the cell walls obtained from 

 a liter of culture containing 10^ cell per milliliter would 

 contain only lO^^ acceptor sites or 0.01 ^^M of acceptor per 

 liter of culture." ^^ This problem of experimental demon- 

 stration of the transfer of the obvious intermediates into 

 wall is a difficult one and indeed seems to be general to the 

 whole problem of the synthesis of large polymers including 

 cellulose."^^ However, some ideas of the mechanisms of wall 

 biosynthesis are emerging from pioneer work of Strominger 

 and his colleagues. 



The biochemical unity of life so admirably discussed by 

 the late Kluyver and Van Niel in their Prather Lectures ^^ 

 prompts me to be optimistic and believe that some of the 

 pathways for wall synthesis recently suggested by Stromin- 

 ger 35 will become established as general mechanisms for the 

 synthesis of these most interesting heteropolymers. These 

 schemes, as Park ^^ has also pointed out, involve a transfer 



