EVIDENCE FOR A POLYMERIC INTERMEDIATE IN CELLULOSE BIOSYNTHESIS

Abstract

Most of the radioactive glucose incorporated into the α-cellulose of sugar beet and barley leaves during short-term photosynthesis in C14O2can be removed in a small proportion of weight by digesting the cellulose in hot, dilute trichloroacetic acid solution. This acid-soluble component contains little or no free labelled glucose. The radioactivity resides in a glucosan of sufficiently high degree of polymerization that it readily and irreversibly precipitates from solution. The radioactive glucosan is resistant to acid hydrolysis but is readily hydrolyzed by cellulase.While the total radioactivity in the α-cellulose fraction does not increase significantly after 10 minutes following cessation of C14O2administration, a slow transfer of radioactivity from the acid-soluble component to the acid-insoluble residual cellulose can be measured over a period of several hours. The transfer is hastened by holding the leaves in darkness. Both the initial incorporation and the subsequent transfer are blocked by physical alteration of the leaf structure through grinding or freezing and thawing. These results are interpreted as evidence for the participation of a polymeric glucosan as an intermediate in the biosynthesis of leaf cellulose.