CELL-TYPE SPECIFICITY OF THE ANABAENA FDXN-ELEMENT REARRANGEMENT REQUIRES XISH AND XISI

The fdxN element, along with two other DNA elements, is excised from t he chromosome during heterocyst differentiation in Anabaena sp. strain PCC 7120. Previous work showed that rearrangement of the fdxN element requires the xisF gene, which encodes a site-specific recombinase, an d suggested that at least one other heterocyst-specific factor is invo lved. Here we report that the xisH and xisI genes are necessary for th e heterocyst-specific excision of the fdxN element. Deletion of a 3.2 kb region downstream of the xisF gene blocked the fdxN-element rearran gement in heterocysts. The 3.2 kb deletion was complemented by the two overlapping genes xisH and xisI. Interestingly, extra copies of xisHI on a replicating plasmid resulted in the xisF-dependent excision of t he fdxN element in vegetative cells. Therefore, xisHI are involved in the control of cell-type specificity of the fdxN rearrangement. The xi sHI genes had no effect on the two other DNA rearrangements. The xisHI -induced excision of the fdxN element produced strains lacking the ele ment and demonstrates that the 55 kb element contains no essential gen es. xisH and xisI do not show similarity to any known genes.