Characterization of genes in the cellulose-synthesizing operon (acs operon) of Acetobacter xylinum: implications for cellulose crystallization

Abstract

The synthesis of an extracellular ribbon of cellulose in the bacterium Acetobacter xylinum takes place from linearly arranged, membrane-localized, cellulose-synthesizing and extrusion complexes that direct the coupled steps of polymerization and crystallization. To identify the different components involved in this process, we isolated an Acetobacter cellulose-synthesizing (acs) operon from this bacterium. Analysis of DNA sequence shows the presence of three genes in the acs operon, in which the first gene (acsAB) codes for a polypeptide with a molecular mass of 168 kDa, which was identified as the cellulose synthase. A single base change in the previously reported DNA sequence of this gene, resulting in a frameshift and synthesis of a larger protein, is described in the present paper, along with the sequences of the other two genes (acsC and acsD). The requirement of the acs operon genes for cellulose production was determined using site-determined TnphoA/ Kanr GenBlock insertion mutants. Mutant analysis showed that while the acsAB and acsC genes were essential for cellulose production in vivo, the acsD mutant produced reduced amounts of two cellulose allomorphs (cellulose I and cellulose II), suggesting that the acsD gene is involved in cellulose crystallization. The role of the acs operon genes in determining the linear array of intramembranous particles, which are believed to be sites of cellulose synthesis, was investigated for the different mutants; however, this arrangement was observed only in cells that actively produced cellulose microfibrils, suggesting that it may be influenced by the crystallization of the nascent glucan chains.Cellulose is an extracellular polysaccharide, synthesized as long f-1,4 glucan chains that associate to form the microfibrils commonly observed in cellulose-synthesizing organisms. The synthesis of this biopolymer takes place via a single polymerization step, utilizing UDP-glucose as the substrate and-catalyzed by the enzyme cellulose synthase (UDP-glucose: 1,4-p-D-glucosyltransferase), without the apparent involvement of any intermediates (40). However, the crystalline structure of cellulose microfibrils and their association with organized intramembranous particles observed in most cellulose-synthesizing organisms suggest a more intricate mechanism of cellulose biogenesis than that deduced from the polymerization reaction alone. Because of their association with cellulose microfibrils, the intramembranous particles are believed to be sites of cellulose synthesis and extrusion. The nature of these particles is incompletely understood at present, and it is not known at the molecular level how they become organized into structures that are referred to as terminal synthesizing complexes (TCs) that are characteristic for most organisms (5). In higher plants, the TC is arranged as a rosette; in a large number of algal species, the TC is arranged as a linear row(s) of particles (7). Even though the cellulose produced by all organisms chemically is the same in its primary compositi...

Cellulose Expression in Pseudomonas fluorescens SBW25 and Other Environmental Pseudomonads

“…Overall, the wss operon showed strong similarity to the cellulose biosynthetic clusters originally identified as the acs operon in Gluconacetobacter hansenii (formerly Acetobacter xylinus) ATCC 23769 [54] and subsequently annotated as the yhj operon in the whole-genome sequence of Escherichia coli K-12 [55]. Most acs (Acetobacter cellulose-synthesizing) homologues are now referred to as bcs (bacterial cellulose synthesizing) genes as we do here (yhj has no meaning).…”

“…G. xylinus produces two crystalline allomorphs, known as cellulose I and II, which requires the cellulose synthase-associated BcsD subunit that couples cellulose polymerisation and crystallization [54,65]. However, SBW25 lacks a BcsD homologue and therefore can only produce non-crystalline amorphous cellulose.…”

The effect of phytohormones on the growth, cellulose production and pellicle properties of Gluconacetobacter xylinus ATCC 53582

“…The cellulose biofilm produced by this organism has been the subject of intense investigation due to its purity, mechanical strength, and high crystallinity. [3][4][5][6][7][8][9][10][11] These properties make bacterial cellulose useful for a variety of industrial applications. 3,[12][13][14][15] Despite decades of study, the complete cellulose biosynthetic pathway has not yet been elucidated; there is still much to learn about its regulation, export mechanisms and assembly into the crystallized form.…”

Higher plants contain homologs of the bacterial celA genes encoding the catalytic subunit of cellulose synthase.

“…2 and 3). Furthermore, an operon of four genes involved in cellulose synthesis in A. xylinum has been cloned (4)(5)(6)(7).…”