Advanced Regulatory Topics

Summary

There are many potential problems inherent in trying to regulate multioperon systems. Commonly used strategies include activator proteins that must be present at every promoter in every operon within the system and special sigma factors that activate the otherwise inactive promoters. It is becoming clear that small RNA molecules may also regulate either positively or negatively. As in the case of phage T4, bacteria also sometimes use cascades of sigma factors in which each new sigma factor induces synthesis of yet another sigma factor. The advantage of such a system is its ability to ensure that specific biochemical events occur in a defined sequence. An additional regulatory element that sometimes occurs both in prokaryotes and in eukaryotes is the enhancer, a discrete site to which a protein must bind for a promoter to be fully functional. Multifunctional regulatory proteins are often the subjects of intense study by geneticists because their distinct functions are associated with specific nonoverlapping domains within the molecule. Experimenters can use site-specific mutagenesis to eliminate only one function while preserving others. Examples of genes kept nonfunctional by insertion of extraneous genetic information are now known. Removal of the extra material can occur by protein splicing or recombination prior to transcription.