Transcription -
A
gene
is
transcribed
into
RNA
by
a
DNA-dependent
RNA
polymerase,
which
copies
a
single
stranded
DNA
template
using
the
four
ribonucleotide
triphosphates
as
substrates.
Plant
cells,
in
common
with
animal
cells,
contain
three
types
of
RNA
polymerase
which
can
be
distinguished
on
the
basis
of
physical
properties,
localization
and
function.
RNA
polymerase
II
is
responsible
for
the
synthesis
of
mRNA
precursors
and
is
located
in
the
nucleoplasm.
RNA
polymerases
have
been
isolated
and
characterized
from
a
wide
range
of
plant
species;
they
carry
out
the
first
vital
step
in
gene
expression.
RNA
polymerase
I
synthesizes
only
ribosomal
RNA;
RNA
polymerase
III
synthesizes
tRNAs
and
some
other
small
RNAs.
RNA
polymerase,
II
is
responsible
for
the
synthesis
of
RNA
from
structural
genes.
Promoter
regions
for
RNA
polymerase
II
have
been
estimated
from
a
comparison
of
the
base
sequences
upstream
from
many
different
eukaryotic
genes.
There
are
two "consensus" regions
that
are
good
candidates
for
promoters.
