Human Calmodulin and Troponin C
Genetic Organization

Within the human genome there are three copies of the calmodulin gene on three different chromosomes: 14, 2 and 19 for CALM 1, CALM 2, and CALM 3 respectively. As in most eukaryotic genes, the 450 DNA bases pairs needed to encode calmodulin are divided up into several coding units called exons and spread over a DNA sequence that contains about 10,000 base pairs. The calmodulin gene has 6 exons, but only Exons 2-5 are used for synthesis of the protein. The parts of the protein encoded by each exon are indicated in the figure below.

The gene structure for Troponin CS is exactly like that of Calmodulin: six exons, of which only the last five are used for making the protein. In the five exons that encode the protein, the breaks come in the same places. It seems clear that both proteins had an ancestral protein in common. However, there is only a single copy of the Troponin CS gene in the human genome.

The three CALM genes encode virtually identical proteins but the genes themselves are only about 85% identical. This is taken to mean that there is strong selective pressure to maintain the calmodulin sequence as it is. There are also five nonfunctional pseudogenes. It is assumed that all of the calmodulin genes are copies of some original sequence. The protein is extremely conservative; human calmodulin is identical to the calmodulins of rabbits, salmon, sea urchins and many other animals.
Click here to hear human and sea anemone calmodulin played together. .

One of the things we see in the recently completed draft of the human genome is that this sort of widespread dispersal of similar sequences is common. In some parts of the human genome copies of whole strings of genes are found on different chromosomes.

Sequences used in this presentation were obtained from the SwissProt data base.