Excision of a 37-kb excision element as a circular plasmid in the cyanobacterium Anabaena variabilis

Abstract Anabaena (aka Trichormus ) variabilis ATCC 29413 strain FD is a filamentous, heterocyst-forming cyanobacterium with a 6.36 Mb chromosome, three circular plasmids, A (366 kb), B (35.8 kb), C (301 kb), and a 37-kb excision element. This 37-kb element in A. variabilis ATCC 29413 strain FD was integrated into the tRNA cys gene but was absent in the very closely related strains A. variabilis FSR and PNB. The 37-kb element was also excised as a circular molecule at a very low frequency compared to the integrated form. The 37-kb element has a partial copy of tRNA cys ; therefore, integration produced a functional but genetically distinct copy of tRNA cys . Integration and excision are likely mediated by a putative integrase with a tyrosine recombinase domain encoded within the element, possibly using the duplicated copies of the 7-bp anticodon of the tRNA cys as a recombination site. Like many other bacterial elements, the function of this element is unknown, but, like other well-characterized excision elements in heterocystous cyanobacteria, it does not appear to be important for survival.