13
Biosynthesis of Bacterial Cellulose
synthesis.of.polysaccharide.chains..At.the.same.time,.similar.to.these.polymers,.a.single.polymerizing.enzyme.is.sufficient.for.the.processive.synthesis.
nism.by.which.β-1,4-glucan.chain.initiation.occurs.during.cellulose.synthesis.in.bacteria.is.not.known,.but.it.is.evident.from.in.vitro.cellulose.synthesis.
experiments. that. the. enzyme. cellulose. synthase. is. sufficient. for. initiation.
and. elongation. of. the. glucan. chains,. and. no. primer. is. required. for. chain.
thase.during.cellulose.synthesis.and.not.much.is.known.about.the.mechanism. of. chain. termination.. During. cellulose. synthesis. by. living. cells,. the.
cellulose.microfibrils.may.either.form.a.network,.as.in.the.case.of.G. xylinus,.
or.associate.with.other.components.of.the.cell.wall,.as.in.the.case.of.plants..
In.either.case,.the.cellulose.product.gets.fixed,.and.in.the.case.of.bacteria,.the.
cells.may.move.during.cellulose.synthesis.until.they.get.trapped.in.the.cellulose.network,.whereas.in.the.case.of.plants.it.is.the.cellulose.synthases.that.
move. within. the. plane. of. the. plasma. membrane. (Brown. and. Montezinos.
1976)..It.is.quite.likely.that.chain.termination.may.result.due.to.a.physical.
breakage.of.the.microfibrils.from.the.cells.when.either.the.bacterial.cells.or.
the.cellulose.synthases.in.plant.cells.are.no.longer.able.to.move.
Although.we.can.debate.whether.the.repeating.subunit.in.the.β-1,4-glucan.
chains.in.cellulose.is.glucose.or.cellobiose,.there.is.no.doubt.about.the.two-fold.
symmetry. of. these. chains.. The. mechanism. by. which. the. adjacent. glucose.
residues.assume.a.180°.rotation.with.respect.to.each.other.in.the.β-1,4-glucan.
chains.is.not.clearly.understood.and.a.model.proposing.two.UDP-glucose.
binding.sites.in.the.cellulose.synthase.suggested.that.this.feature.of.the.glucan.chains.is.generated.during.cellulose.synthesis.(Saxena.et.al..1995)..Since.
then,. other. proposals. have. suggested. that. a. dimer. of. cellulose. synthases,.
each.with.a.single.UDP-glucose.binding.site,.may.be.involved.in.generating.
the. 180°. orientation. of. adjacent. glucose. residues,. or. that. a. single. cellulose.
synthase.with.a.single.UDP-glucose.binding.site.can.add.glucose.residues.
to.the.growing.end.and.these.residues.would.then.be.able.to.rotate.on.their.
own.once.they.have.exited.the.catalytic.site.(Delmer.1999)..Unfortunately.the.
crystallographic.structure.of.cellulose.synthase.remains.to.be.determined,.
and.in.its.absence.this.question.remains.unanswered.
Many Bacteria Have Genes for Cellulose Synthesis,
but Only a Few Produce Cellulose
Cellulose.production.by.Bacterium xylinum.was.described.by.A..J..Brown.in.
1886,.almost.50.years.after.the.first.description.of.cellulose.from.plant.. tissues.
by. Anselm. Payen. in. 1838.. G. xylinus. has. been. the. organism. of. choice. for.
Biosynthesis of Bacterial Cellulose
synthesis.of.polysaccharide.chains..At.the.same.time,.similar.to.these.polymers,.a.single.polymerizing.enzyme.is.sufficient.for.the.processive.synthesis.
nism.by.which.β-1,4-glucan.chain.initiation.occurs.during.cellulose.synthesis.in.bacteria.is.not.known,.but.it.is.evident.from.in.vitro.cellulose.synthesis.
experiments. that. the. enzyme. cellulose. synthase. is. sufficient. for. initiation.
and. elongation. of. the. glucan. chains,. and. no. primer. is. required. for. chain.
thase.during.cellulose.synthesis.and.not.much.is.known.about.the.mechanism. of. chain. termination.. During. cellulose. synthesis. by. living. cells,. the.
cellulose.microfibrils.may.either.form.a.network,.as.in.the.case.of.G. xylinus,.
or.associate.with.other.components.of.the.cell.wall,.as.in.the.case.of.plants..
In.either.case,.the.cellulose.product.gets.fixed,.and.in.the.case.of.bacteria,.the.
cells.may.move.during.cellulose.synthesis.until.they.get.trapped.in.the.cellulose.network,.whereas.in.the.case.of.plants.it.is.the.cellulose.synthases.that.
move. within. the. plane. of. the. plasma. membrane. (Brown. and. Montezinos.
1976)..It.is.quite.likely.that.chain.termination.may.result.due.to.a.physical.
breakage.of.the.microfibrils.from.the.cells.when.either.the.bacterial.cells.or.
the.cellulose.synthases.in.plant.cells.are.no.longer.able.to.move.
Although.we.can.debate.whether.the.repeating.subunit.in.the.β-1,4-glucan.
chains.in.cellulose.is.glucose.or.cellobiose,.there.is.no.doubt.about.the.two-fold.
symmetry. of. these. chains.. The. mechanism. by. which. the. adjacent. glucose.
residues.assume.a.180°.rotation.with.respect.to.each.other.in.the.β-1,4-glucan.
chains.is.not.clearly.understood.and.a.model.proposing.two.UDP-glucose.
binding.sites.in.the.cellulose.synthase.suggested.that.this.feature.of.the.glucan.chains.is.generated.during.cellulose.synthesis.(Saxena.et.al..1995)..Since.
then,. other. proposals. have. suggested. that. a. dimer. of. cellulose. synthases,.
each.with.a.single.UDP-glucose.binding.site,.may.be.involved.in.generating.
the. 180°. orientation. of. adjacent. glucose. residues,. or. that. a. single. cellulose.
synthase.with.a.single.UDP-glucose.binding.site.can.add.glucose.residues.
to.the.growing.end.and.these.residues.would.then.be.able.to.rotate.on.their.
own.once.they.have.exited.the.catalytic.site.(Delmer.1999)..Unfortunately.the.
crystallographic.structure.of.cellulose.synthase.remains.to.be.determined,.
and.in.its.absence.this.question.remains.unanswered.
Many Bacteria Have Genes for Cellulose Synthesis,
but Only a Few Produce Cellulose
Cellulose.production.by.Bacterium xylinum.was.described.by.A..J..Brown.in.
1886,.almost.50.years.after.the.first.description.of.cellulose.from.plant.. tissues.
by. Anselm. Payen. in. 1838.. G. xylinus. has. been. the. organism. of. choice. for.