The immonoblot procedure was carried out according to the manufacturer’s instructions (GE Healthcare). The GFP antibody [Anti-GFP, rabbit IgG fraction (Invitrogen)] was used at a 1 : 5000 dilution. The secondary antibody [Immun-Star Goat Anti-Rabbit (GAR)–HRP Conjugate (Bio-Rad)] was used at a 1 : 5000 dilution. Detection was performed using Immun-Star HRP Substrate (Bio-Rad), and recorded using a ChemiDoc

XRS system (Bio-Rad). SDS-PAGE Western blots were performed with biological triplicates. For TEM, heterocysts were fixed and treated as described by Bergman et al. (1985). Ultrathin sections were examined by Zeiss Supra35-VP Field Emission SEM, equipped with a STEM Detector (see Fig. S2 for details). Using OE-PCR, a gfp-modified version of the complete N. punctiforme hup-operon with the Dapagliflozin manufacturer insertion of a sequence coding for a proline–threonine linker and a gfp coding sequence, enabling expression of a HupS–GFP fusion protein, was constructed. This construct was cloned into the pSUN119 shuttle vector to generate HCS assay plasmid pSHG (Fig. 1a). In the N2-fixing SHG cultures, Western blotting showed a GFP band corresponding to the size of the HupS–GFP fusion protein (62.5 kDa), along with a minority (variable amount, always in minority of total bands) of degradation products all larger in size than GFP (27 kDa).

No GFP bands were found in the non-N2-fixing SHG cultures (Fig. 1b) or in the WT controls (data not shown). To determine the cellular localization of HupS–GFP in the filaments, SHG and WT cultures were examined using laser scanning confocal microscopy before and at different time-points after nitrogen depletion. Neither GFP fluorescence nor heterocysts were observed

in any culture before nitrogen depletion. After 24 h of combined nitrogen starvation, lower red autofluorescence (compared with vegetative cells), and a weak GFP fluorescence in SHG, could be observed in proheterocysts (data not shown). After 34 h, the filaments had developed mature heterocysts with low red auto fluorescence (compared with vegetative cells) and a strong GFP fluorescence in SHG (Fig. 2). No GFP signal was observed from any of the non-N2-fixing cultures, the vegetative cells of the N2-fixing SHG cultures or from N2-fixing WT cultures Mephenoxalone (Fig. 2). To investigate the subcellular localization of HupS–GFP in the heterocysts, SHG was examined before nitrogen depletion, and at different time-points after initiation of combined nitrogen starvation. The proheterocysts observed 24 h after nitrogen depletion had a weak and homogeneously distributed GFP fluorescence (data not shown). After about 30 h and up to 1 week after nitrogen depletion (longest time tested), fully developed heterocysts were observed. The GFP fluorescence at the later time points (30 h and longer) was either homogeneously distributed or localized in several smaller or fewer larger clusters (Fig. 3a).