A histidine residue, located in the C-terminal section of the enzyme, plays a central role in its catalytic mechanism.

The crystal structure of the type III enzyme from Escherichia coli with chloramphenicol bound has been determined.

His195 is appropriately positioned to act as a general base catalyst in the reaction, and the required tautomeric stabilisation is provided by an unusual interaction with a main-chain carbonyl oxygen.

[3] CAT is used as a reporter system to measure the level of a promoter or its tissue-specific expression.

The CAT assay involves monitoring acetylation of radioactively labeled chloramphenicol on a TLC plate; CAT activity is determined by looking for the acetylated forms of chloramphenicol, which have a significantly increased migration rate as compared to the unacetylated form.