Mechanism of cellulose synthesis in Agrobacterium tumefaciens

Abstract

Extracts of Agrobacterium tumefaciens incorporated UDP-[14C]glucose into cellulose. When the extracts were fractionated into membrane and soluble components, neither fraction was able to synthesize cellulose. A combination of the membrane and soluble fractions restored the activity found in the original extracts. Extracts of cellulose-minus mutants showed no significant incorporation of UDP-glucose into cellulose. When mixtures of the extracts were made, the mutants were found to fall into two groups: extracts of mutants from the first group could be combined with extracts of the second group to obtain cellulose synthesis. No synthesis was observed when extracts of mutants from the same group were mixed. The groups of mutants corresponded to the two operons identified in sequencing the cel genes (A. G. Matthysse, S. White, and R. Lightfoot. J. Bacteriol. 177:1069-1075, 1995). Extracts of mutants were fractionated into membrane and soluble components, and the fractions were mixed and assayed for the ability to synthesize cellulose. When the membrane fraction from mutants in the celDE operon was combined with the soluble fraction from mutants in the celABC operon, incorporation of UDP-glucose into cellulose was observed. In order to determine whether lipid-linked intermediates were involved in cellulose synthesis, permeablized cells were examined for the incorporation of UDP-[14C]glucose into material extractable with organic solvents. No radioactivity was found in the chloroform-methanol extract of mutants in the celDE operon, but radioactive material was recovered in the chloroform-methanol extract of mutants in the celABC operon. The saccharide component of these compounds was released after mild acid hydrolysis and was found to be mainly glucose for the celA insertion mutant and a mixture of cellobiose, cellotriose, and cellotetrose for the celB and celC insertion mutants. The radioactive compound extracted with chloroform-methanol form the celC insertion mutant was incorporated into cellulose by membrane preparations from celE mutants, which suggests that this compound is a lipid-linked intermediate in cellulose synthesis.