The smaller chromosome also carries an integron gene capture system (see Chapter 25: Mobile DNA) and hosts “addiction” genes that are typically found on plasmids (discussed later). Many of these genes lack homology to characterized genes and are of unknown function. Many genes that appear to have origins outside the enteric bacteria, as deduced from their different base composition, were found on the smaller chromosome. Together, this totals approximately 4 million base pairs and encodes about 3900 proteins-about the same amount of genetic information as E. By controlling the level of transcription, this process can determine when and how much protein product is made by a gene.When the genome of the Gram-negative bacterium Vibrio cholerae, the causative agent of cholera, was sequenced, it was found to consist of two circular chromosomes of 2,961,146 and 1,072,314 bp. These proteins bind to regulatory regions of a gene and increase or decrease the level of transcription. Signals from the environment or from other cells activate proteins called transcription factors. Gene regulation can occur at any point during gene expression, but most commonly occurs at the level of transcription (when the information in a gene’s DNA is passed to mRNA). Although we know that the regulation of genes is critical for life, this complex process is not yet fully understood. Gene regulation also allows cells to react quickly to changes in their environments.

Genes are turned on and off in different patterns during development to make a brain cell look and act different from a liver cell or a muscle cell, for example. Gene regulation is an important part of normal development. The process of turning genes on and off is known as gene regulation. The rest of the genes are repressed, or turned off. Each cell expresses, or turns on, only a fraction of its genes at any given time.

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