EXAMPLE FOR CONTROL AT THE LEVEL OF ELONGATION
Regulation of elongation: additional mechanism of control for gene expression.
therefore called transcriptional attenuation.
Used only for limited number of genes, best described for E. coli trp operon.
Trp stands for tryptophan: operon contains 5 structural genes important in the synthesis of the amino acid tryptophan.
Tryptophan is only synthesized in E. coli, if it is not available in the food source.
Regulation of the trp operon at the level of initiation of transcription
Initiation of transcription blocked when abundant tryptophan is available:
High level of tryptophan ---- tryptophan can bind to repressor ---- repressor is now able to bind to trp operon and inhibit initiation of transcription (Fig. 6.11).
But transcriptional initiation never totally blocked. Transcriptional attenuation increases stringency of inhibition.
Mechanism of transcriptional attenuation: (Fig. 6.12) animation later
Requires a specific regulatory sequence called attenuator. It lies somewhere downstream of the transcription start site in the transcribed and translated region of the operon.
The attenuator contains a premature termination signal (a GC-rich inverted repeat followed by adenines, sequences 3 and 4 in Figure). If transcription and translation proceed normally (under high tryptophan levels) the mRNA is terminated prematurely and no complete protein can be synthesized.
However, under low levels of tryptophan premature termination is inhibited because the stem loop cannot form from this inverted repeat
----- there is another GC-rich sequence complementary to the first repeat in front of the termination signal (sequence 2 in Figure). This sequence takes part in forming an alternative stem loop, which does not lead to premature termination because it is not followed by a strech of adenines.
Why does this alternative stem loop not form normally? Because if translation occurs with normal speed, the ribosome will already cover the alternative complementary GC-rich sequence and prevent alternative stem loop formation.
Under low tryptophan levels translation is slowed at an earlier sequence of the mRNA (sequence 1 in Figure) simply because it contains codons for tryptophan, and tryptophan is scarce for translation.
So the level of tryptophan directly determines the speed of translation, which in turn determines the kind of stem loop that is formed, either the one that can prematurely terminate transcription (high tryptophan levels) or the one that cannot (low tryptophan levels).
PLASMIDS
The next three subjects: plasmids, bacteriophages, and transposons will show that the genetic make-up of a cell can be subject to changes in addition to those processes we talked about that lead to mutations and rearrangements.
Plasmids: naturally occurring extrachromosomal nucleic acids capable of independent replication in the cell (contain origins of replication)
The overwhelming majority: circular, double-stranded DNA molecules
Plasmids could be found in most bacterial species, but also in some eukaryotes including yeast, some protozoa, and some plants.
We will restrict our discussion to plasmids found in bacteria.
The size of bacterial plasmids ranges from a few kb to more than 500 kb, or from encoding one or two genes to several hundred genes.
The genes encoded by a plasmid often provide an additional advantage for the host cell like allowing it to grow in otherwise hostile environments. Examples are genes that confer resistance to antibiotics or to heavy metals, such as mercury and lead.
In the absence of selective pressure plasmids are usually dispensable to the host cell or can even be regarded as a burden because resources must be spent in replicating them.
Some bacterial plasmids have the ability to transfer themselves from one cell to another, thus spreading horizontally in a population. (Vertical spreading would be through growth and cell division.) The ability can go even so far that some plasmids transfer themselves from one bacterial species to another.