(iii) Integrator gene. It is comparable to regulator gene and is responsible for synthesis of an activator RNA that may or may not give rise to proteins before it activates the receptor site (Fig. 37.1). (iv) Sensor site. A sensor site regulates the activity of integrator gene, which can be transcribed only when the sensor site is activated. The sensor sites are recognized by agents which like hormones and proteins change the pattern of gene expression. For instance, hormone-protein complex or a transcription factor may bind to a sensor site and cause the transcription of integrator.

Fig. 37. 1. Different components in Britten-Davidson's model.

The inter-relationship of the four classes of sequences given above are illustrated in Figure 37.1. In this model, the genes (producer gene and integrator gene) are those sequences which are involved in RNA synthesis. On the other hand, sites (receptor and sensor sites) are those sequences which help only in recognition without taking part in RNA synthesis. It is also proposed that receptor sites and integrator genes may be repeated a number of times, so as to control the activity of a large number of genes in the same cell. Repetition of receptor ensures that same activator recognizes all of them and several enzymes of one pathway are simultaneously synthesized.

It is also possible that the transcription of same gene be needed in different developmental stages. This can be achieved by multiplicity of receptor sites and integrator genes. Each structural gene may have several receptor sites, each responding to one activator (Fig. 37.2) so that a single activator though can recognize several genes, but different activators may activate the same gene at different times. An integrator gene may also fall in cluster with same sensor site (Fig. 37.3).

Fig. 37.2. Redundancy of receptor sites (as proposed in Britten-Davidson's model).

Fig. 37.3. Redundancy of integrator genes (as proposed in Britten-Davidson's model).