or downstream of a particular gene in an orientation-independent manner. Even at
such great distances from the transcription start point they may increase transcription
by several hundred-fold. The precise interactions between transcription factors, RNA
polymerase or other DNA binding proteins and the DNA sequences they bind to may
be identified and characterised by the technique of DNA footprinting (Section 6.8.3).
For transcription in eukaryotic cells to proceed a number of transcription factors need
to interact with the promoters and with each other. This cascade mechanism is
indicated in Fig. 5.15c and is termed apre-initiation complex. Once this has been
formed around the35 TATA sequence RNA polymerase II is able to transcribe the
structural gene and form a complementary RNA copy (Section 5.5.6).5.5.5 Transcription in prokaryotes
Prokaryotic gene organisation differs from that found in eukaryotes in a number of
ways. Prokaryotic genes are generally found as continuous coding sequences which
are not interrupted. Moreover they are frequently found clustered intooperons
which contain genes that relate to a particular function such as the metabolism of a
substrate or synthesis of a product. This is particularly evident in the best-known
operon identified inE. colitermed thelactose operonwhere three geneslacZ,lacY
andlacAshare the same promoter and are therefore switched on and off at the same
time. In this model the absence of lactose results in a repressor protein binding to an
operator region upstream of theZ,YandAgene and prevents RNA polymerase
from transcribing the genes (Fig. 5.16a). However the presence of lactose requires
the genes to be transcribed to allow its metabolism. Lactose binds to the repressor
protein and causes a conformational change in its structure. This prevents it binding
to the operator and allows RNA polymerase to bind and transcribe the three genes
(Fig. 5.16b). Transcription and translation in prokaryotes is also closely linked or
coupled whereas in eukaryotic cells the two processes are distinct and take place in
different cell compartments.5.5.6 Post-transcriptional processing
Transcription of a eukaryotic gene results in the production of a heterogeneous
nuclear RNA transcript (hnRNA) which faithfully represents the entire structural gene
(Fig. 5.17). Three processing events then take place. The first processing step involves
the addition of a methylated guanosine residue (m7Gppp) termed a cap to the 5^0 end
of the hnRNA. This may be a signalling structure or aid in the stability of the mole-
cule (Fig. 5.18). In addition, 150 to 300 adenosine residues termed apoly(A) tail
are attached at the 3^0 end of the hnRNA by the enzyme poly(A) polymerase. The
poly(A) tail allows the specific isolation of eukaryotic mRNA from total RNA by
affinity chromatography (Section 5.7.2); its presence is thought to confer stability
on the transcript.
Unlike prokaryotic transcripts those from eukaryotes have their coding sequence
(expressed regions or exons) interrupted by non-coding sequence (intervening157 5.5 Functions of nucleic acids