Two DNA elements are excised from the chromosome during Anabaena heter
ocyst differentiation. We have identified the gene xisF which encodes
the site-specific recombinase responsible for the excision of a 55-kb
element from within the fdxN gene. The cloned xisF gene is sufficient
to cause site-specific rearrangement of an artificial substrate in Esc
herichia coli. Inactivation of xisF in the Anabaena chromosome prevent
s excision of the fdxN element and growth in nitrogen-deficient medium
but does not alter the development of heterocysts. Forced transcripti
on of xisF in vegetative cells did not result in excision of the fdxN
element, suggesting that other factors may be involved in cell-type sp
ecificity. The predicted XisF protein shows significant similarity to
the Bacillus subtilis SpoIVCA recombinase.