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

Some vegetative cells of 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 N-fixation, is it necessary for a heterocyst to retain the entire genome (≈7.1 Mbp) from its progenitor vegetative cell? By sequencing the heterocyst genome, we discovered and confirmed that at least six DNA elements (≈0.12 Mbp) are deleted during heterocyst development. The six-element deletions led to the restoration of five genes (, , , 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 genes (, and ) from N-fixing cyanobacteria (multicellular and unicellular) as well as other N-fixing bacteria (non-cyanobacteria), we found that interrupted genes all contain the conserved core sequences that may be required for phage DNA insertion. Here, we discuss the 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 N-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.