AGGA Box -

Another sequence that may be involved in regulation of transcription of some eukaryotic genes is the consensus sequence "CAAT" box or GG(CT)CAATCT. This sequence is often found -40 and -180 nuleotide upstream of the cap site. The sequence of this element is much more variable but often includes the sequence CAAT, termed the "CAAT Box". In plants it is referred to as the AGGA box.

Two zein genomic clones, Z4 and Z7, have sequences with limited homology to the CAAT box. The leghemoglobin genes have three sequences upstream of the coding region, all with some homology to the animal sequence. All of the plant sequences have an interesting symmetry of adenines surrounding the trinucleotide (G/T)NG.

Although a few cases are known wherein regulatory elements occur in the gene itself (a prime example is 5S rRNA genes transcribed by RNA polymerase III), transcriptional responses usually seem to be determined by the sequences5' to the start of transcription, often called 5'-flanking sequences.

A good example is provided by the heat shock genes which are activated when cells are subjected to thermal stress. In both animals and plants it has been Possible to show that the 5' flanking sequences from a heat shock gene can confer responsiveness to a gene not normally responsive to heat shock.

This element seems to be required for responsiveness to heat shock and is located in a region which can be shown by direct binding experiments to interact with a protein transcription factor. For example, in Drosophila the flanking sequences of different heat shock genes contain a common element that is 14 nucleotides long and located 11-28 bases upstream from the TATA box.

Indeed, the ability to influence gene expression from different positions and orientations is the main diagnostic feature of enhancer elements. Although most enhancer elements are found in flanking sequences on the 5'-ends of genes, they are also found to occur elsewhere.

An "enhancer core" sequence such as GGTGTGGAAAG, or more generally GTGGT/AT/AT/AG ('II A' means that either T or A is found in this position), can be identified by sequence comparisons, although it is clear that sequences other than the core are also important.

Enhancer sequences are often associated with regions of alternating purines and pyrimidines which are capable of forming Z-DNA. Enhancer like sequences in plants stimulate the expression of genes such as RuBP carboxylase in cells grown in light but not those grown in the dark.