Spatial fluctuations in expression of the heterocyst differentiation regulatory gene hetR in Anabaena filaments

PLoS Genet. 2015 Apr 1;11(4):e1005031. doi: 10.1371/journal.pgen.1005031. eCollection 2015 Apr.

Abstract

Under nitrogen deprivation, filaments of the cyanobacterium Anabaena undergo a process of development, resulting in a one-dimensional pattern of nitrogen-fixing heterocysts separated by about ten photosynthetic vegetative cells. Many aspects of gene expression before nitrogen deprivation and during the developmental process remain to be elucidated. Furthermore, the coupling of gene expression fluctuations between cells along a multicellular filament is unknown. We studied the statistics of fluctuations of gene expression of HetR, a transcription factor essential for heterocyst differentiation, both under steady-state growth in nitrogen-rich conditions and at different times following nitrogen deprivation, using a chromosomally-encoded translational hetR-gfp fusion. Statistical analysis of fluorescence at the individual cell level in wild-type and mutant filaments demonstrates that expression fluctuations of hetR in nearby cells are coupled, with a characteristic spatial range of circa two to three cells, setting the scale for cellular interactions along a filament. Correlations between cells predominantly arise from intercellular molecular transfer and less from cell division. Fluctuations after nitrogen step-down can build up on those under nitrogen-replete conditions. We found that under nitrogen-rich conditions, basal, steady-state expression of the HetR inhibitor PatS, cell-cell communication influenced by the septal protein SepJ and positive HetR auto-regulation are essential determinants of fluctuations in hetR expression and its distribution along filaments. A comparison between the expression of hetR-gfp under nitrogen-rich and nitrogen-poor conditions highlights the differences between the two HetR inhibitors PatS and HetN, as well as the differences in specificity between the septal proteins SepJ and FraC/FraD. Activation, inhibition and cell-cell communication lie at the heart of developmental processes. Our results show that proteins involved in these basic ingredients combine together in the presence of inevitable stochasticity in gene expression, to control the coupled fluctuations of gene expression that give rise to a one-dimensional developmental pattern in this organism.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Anabaena / genetics*
  • Anabaena / growth & development
  • Anabaena / metabolism
  • Bacterial Proteins / genetics
  • Bacterial Proteins / metabolism
  • Gene Expression Regulation, Bacterial*
  • Gene Expression Regulation, Developmental*
  • Periodicity
  • Protein Transport
  • Transcription Factors / genetics
  • Transcription Factors / metabolism*

Substances

  • Bacterial Proteins
  • FraC protein, Anabaena
  • PatS protein, Anabaena
  • Transcription Factors

Grant support

LCG was the recipient of a JAE predoc fellowship from the CSIC. JS is the incumbent of the Siegfried and Irma Ullman Professorial Chair. Work in Seville was supported by grants BFU2011-22762 (EF) and BFU2013-44686-P (AH) from Plan Nacional de Investigación, Desarrollo e Investigación, Spain, co-financed by the European Regional Development Fund. Work in Rehovot was supported by the Minerva Foundation with funding from the Federal German Ministry for Education and Research (JS) (http://www.minerva.mpg.de/). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.