Repeated Sequences of Regulating Gene Expression -

In a strict sense, some of the control sequences just described is really repeated sequences. Short elements, such as the TATA or CAAT boxes, appear in many promoters and hence are repeated, but because they are so short are not recognized in hybridization experiments.

Enhancers may sometimes contain several repeats of a somewhat longer sequence (such as the 72-nucleotide repeats of the SV40 enhancer), By themselves, however, these do not reach sufficiently high copy numbers to be detected in classical reassociation experiments and no extensive sequence homology exists among enhancers as a class, although some short homologies have been found. Therefore, none of the gene regulatory elements discussed thus far can be considered "classical" repeated sequences.

On the other hand, if must be admitted that a small class of functionally important repeat sequences might exist against a large background of evolutionary noise. Several lines of evidence from animal systems suggest that this may be true.

The transcriptional elements are parts of a gene or its flanking sequences and affect only the gene to which they are directly linked. In genetic terms they function cis, regulating genes on the same chromosome. Clearly, there must be other elements involved in gene regulation and, as a matter of fact, there is now good experimental evidence that in some cases certain protein factors must be bound to specific sites in a gene or its flanking sequences in order to initiate transcription.