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

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 tRNAcys 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 tRNAcys; therefore, integration produced a functional but genetically distinct copy of tRNAcys. 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 tRNAcys 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.