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Some vegetative cells ofAnabaena cylindricaare 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 (≈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 (nifH1,nifD,hupL,primase P4and 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 intactnifgenes (nifH,nifDandhupL) from N2-fixing cyanobacteria (multicellular and unicellular) as well as other N2-fixing bacteria (non-cyanobacteria), we found that interruptednifgenes all contain the conserved core sequences that may be required for phage DNA insertion. Here, we discuss thenifgenes 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. |
