Filamentous cyanobacteria of the order Nostocales display typical properties of multicellular organisms. In response to
nitrogen starvation, some vegetative cells differentiate into heterocysts, where fixation of N(2) takes place. Heterocysts provide a micro-oxic compartment to protect
nitrogenase from the
oxygen produced by the vegetative cells. Differentiation involves fundamental remodeling of the gram-negative cell wall by deposition of a thick envelope and by formation of a neck-like structure at the contact site to the vegetative cells. Cell wall-hydrolyzing
enzymes, like cell wall
amidases, are involved in
peptidoglycan maturation and turnover in unicellular bacteria. Recently, we showed that mutation of the
amidase homologue amiC2 gene in Nostoc punctiforme ATCC 29133 distorts filament morphology and function. Here, we present the functional characterization of two amiC paralogues from Anabaena sp. strain PCC 7120. The amiC1 (alr0092) mutant was not able to differentiate heterocysts or to grow diazotrophically, whereas the amiC2 (alr0093) mutant did not show an altered phenotype under standard growth conditions. In agreement, fluorescence recovery after photobleaching (FRAP) studies showed a lack of cell-cell communication only in the AmiC1 mutant.
Green fluorescent protein (GFP)-tagged AmiC1 was able to
complement the mutant phenotype to wild-type properties. The
protein localized in the septal regions of newly dividing cells and at the neck region of differentiating heterocysts. Upon
nitrogen step-down, no mature heterocysts were developed in spite of ongoing heterocyst-specific gene expression. These results show the dependence of heterocyst development on
amidase function and highlight a pivotal but so far underestimated cellular process, the remodeling of
peptidoglycan, for the biology of filamentous cyanobacteria.