ISOLATION AND PROPERTIES OF WALLS 17 



pointed out, small amounts of nucleic acid can be extracted 

 Irom streptococcal (group A) wall preparations. Whether 

 the nucleic acid or nucleic acid derivatives extractable from 

 the wall are associated with it for "biochemical purposes" 

 is not known. Contrary to the earlier views of Stacey/^ it 

 is now generally conceded that the nucleic acids are thus 

 of minor importance in a consideration of the types of 

 structural polymers in cell walls. 



In addition to the nucleic acids, the cellular pigments 

 also appear to be of intracellular origin, and there is no evi- 

 dence of their being associated covalently with structural 

 compounds encountered in nature. Although many pig- 

 mented organisms give wall fractions devoid of pigments, 

 there is a number of instances in which these compounds 

 persist in the wall fraction during isolation. Cell-wall frac- 

 tions of several photosynthetic bacteria contain both carote- 

 noids and photosynthetic pigments, although the latter are 

 obviously much more abundant in the chromatophore frac- 

 tions.^- ^^ The presence of pigments in the wall fractions 

 can, with some justification, be regarded with suspicion, and 

 their presence may be an artifact of the isolation procedures. 

 However, it may well be that in some organisms certain 

 pigments are located in the wall of the intact cell. Isolated 

 walls of the two blue-green algae, Anacystis nidulans and 

 Microcoleus vaginatus, contained carotenoids, but the chlo- 

 rophylls separated in a small particle fraction quite cleanly 

 from the wall fractions (Salton, unpublished data). 



In selecting results to illustrate the general features of the 

 chemistry of cell walls, I have confined my choice largely 

 to studies in which the wall structures have been isolated 

 by mechanical disintegration and differential centrifuga- 

 tion. It became apparent during the search for this data 

 that, apart from bacteria, little information is available for 



