Sigma factor genes sigC, sigE, and sigG are upregulated in heterocysts of the cyanobacterium Anabaena sp. strain PCC 7120

Abstract

We used gfp transcriptional fusions to investigate the regulation of eight sigma factor genes during heterocyst development in the cyanobacterium Anabaena sp. strain PCC 7120. Reporter strains containing gfp fusions with the upstream regions of sigB2, sigD, sigI, and sigJ did not show developmental regulation. Time-lapse microscopy of sigC, sigE, and sigG reporter strains showed increased green fluorescent protein fluorescence in differentiating cells at 4 h, 16 h, and 9 h, respectively, after nitrogen step down.

FIG. 1.

GFP reporter fluorescence from strains containing gfpmut2 expressed from promoters of the sigB2, sigD, sigI, and sigJ sigma factor genes in Anabaena sp. strain PCC 7120 filaments grown in nitrate-containing BG-11 medium and 24 h after nitrogen step-down to BG-11₀ medium to induce heterocyst development. Arrowheads indicate mature heterocysts. The panels on the left for each medium are bright-field images. The panels on the right are corresponding GFP fluorescence images. Scale bar, 10 μm.

FIG. 2.

GFP reporter fluorescence from promoters of sigma factor genes sigC (A), sigE (B), and sigG (C). Upper panels show bright-field images; lower panels show the corresponding GFP fluorescence images. The strains were grown in nitrate-containing BG-11 medium and then transferred to BG-11₀ medium to induce heterocyst development. Time-lapse images were collected every 10 min for at least 24 h, and selected images are shown for the indicated time points after induction. Filaments of the sigC reporter strain were partially fragmented by mild sonication prior to nitrogen step down in the time series shown; similar results were obtained with unfragmented filaments. Scale bar, 10 μm.