In addition to being small, insertion sequences are genetically compact (Fig 1.25.1). They generally encode no functions other than those involved in their
mobility although individual members of several families which include
additional genes are now being identified. IS-encoded functions include factors
required in cis, in particular recombinationally active DNA sequences which
define the ends of the element together with an enzyme, the transposase
(Tpase), which recognises and processes these ends. The Tpase is generally
encoded by a single or perhaps two, open reading frames and consumes nearly the
entire length of the element.