Identification of Nostoc punctiforme akinete-expressed genes using differential display

Akinetes are spore-like resting cells formed by certain filamentous cyanobacteria that have increased resistance to environmental stress. They can be found at low frequencies in dense cultures experiencing low light or phosphate limitation, but also form at high frequencies in a zwf mutant strain of Nostoc punctiforme following dark incubation in the presence of fructose. The wild-type strain is capable of facultative heterotrophic growth under these conditions and does not form akinetes. To identify genes associated with akinete development, differential display was used to amplify and compare cDNA from a wild-type and zwf mutant strain of N. punctiforme following a switch to dark heterotrophic conditions. Screening of candidate genes by reverse transcriptase real-time quantitative PCR and subsequent testing for akinete-specific expression using GFP transcriptional reporter plasmids lead to the identification of three novel akinete-expressed genes. The genes identified from the screening encoded for proteins homologous to an aminopeptidase (aapN), a zinc protease (hap) and an ATP-binding cassette (ABC)-type transporter (aet). Expression of hap was also increased in developing hormogonia, a transient type of differentiated filament capable of gliding motility. Transcriptional start sites for akinete-expressed genes were determined using random amplification of cDNA ends (RACE), and promoter regions were compared with orthologues in other filamentous cyanobacteria to identify putative regulatory sequences.