Developmentally Regulated Genome Editing in Terminally Differentiated N2-Fixing Heterocysts of Anabaena cylindrica ATCC 29414

Some vegetative cells of Anabaena cylindrica are programed to differentiate semi-regularly spaced, single heterocysts along filaments. Since heterocysts are terminally differentiated non-dividing cells, with the sole known function for solar-powered N2-fixation, is it necessary for a heterocyst to retain the entire genome ({approx}7.1 Mbp) from its progenitor vegetative cell? By sequencing the heterocyst genome, we discovered and confirmed that at least six DNA elements ({approx}0.12 Mbp) are deleted during heterocyst development. The six-element deletions led to the restoration of five genes (nifH1, nifD, hupL, primase P4 and a hypothetical protein gene) that were interrupted in vegetative cells. The deleted elements contained 172 genes present in the genome of vegetative cells. By sequence alignments of intact nif genes (nifH, nifD and hupL) from N2-fixing cyanobacteria (multicellular and unicellular) as well as other N2-fixing bacteria (non-cyanobacteria), we found that interrupted nif genes all contain the conserved core sequences that may be required for phage DNA insertion. Here, we discuss the nif genes interruption which uniquely occurs in heterocyst-forming cyanobacteria. To our best knowledge, this is first time to sequence the genome of heterocyst, a specially differentiated oxic N2-fixing cell. This research demonstrated that (1) different genomes may occur in distinct cell types in a multicellular bacterium; and (2) genome editing is coupled to cellular differentiation and/or cellular function in a heterocyst-forming cyanobacterium.