Structural genes controlling expression

From a physiological point of view, uptake systems make sense to a cell only when their substrates are both available in the medium and useful to the cell. As a matter of fact, the expression of the permease structural genes and the activity of the corresponding permeases are controlled and modulated by different types of regulatory systems. 

Figure 1. Hierarchy of control of gene expression. A total of about 50,000 to 100,000 genes are necessary to encode a mammal, most of which encode housekeeping, structural component, or terminal differentiation gene products. Transcription factor genes regulate expression of the lower-level genes and are in turn controlled by other upper-level transcription factors.

Expression of the lac operon is regulated by controlling elements, which are separate from the structural genes. The controlling elements consist of a promoter locus, which is the site where RNA polymerase binds and initiates transcription the promoter locus also contains sites for the binding of a repressor and an activator. The i gene encodes the repressor, and the crp gene encodes the activator. 

Based on these mechanisms a new frontier in the application of enzymes to biotechnology, including the development of synthetic enzymes (synzymes)82,83 will be exploited. Research on structure-function relationships between ribozymes and abzymes will lead to the development of a number of sequence specific catalysts, which will control expression of a specific gene or its products, and eventually to application as pathogen controls in agriculture and to clinical use. Sequence specific abzymes may also facilitate research on the primary structure determination of protein. 

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