The Inorganic Nutrient Regime and the mre Genes Regulate Cell and Filament Size and Morphology in the Phototrophic Multicellular Bacterium Anabaena

Abstract

The model cyanobacterium Anabaena sp. PCC 7120 exhibits a phototrophic metabolism relying on oxygenic photosynthesis and a complex morphology. The organismic unit is a filament of communicated cells that may include cells specialized in different nutritional tasks, thus representing a paradigm of multicellular bacteria. In Anabaena, the inorganic carbon and nitrogen regime influenced not only growth, but also cell size, cell shape, and filament length, which also varied through the growth cycle. When using combined nitrogen, especially with abundant carbon, cells enlarged and elongated during active growth. When fixing N2, which imposed lower growth rates, shorter and smaller cells were maintained. In Anabaena, gene homologs to mreB, mreC, and mreD form an operon that was expressed at higher levels during the phase of fastest growth. In an ntcA mutant, mre transcript levels were higher than in the wild type and, consistently, cells were longer. Negative regulation by NtcA can explain that Anabaena cells were longer in the presence of combined nitrogen than in diazotrophic cultures, in which the levels of NtcA are higher. mreB, mreC, and mreD mutants could grow with combined nitrogen, but only the latter mutant could grow diazotrophically. Cells were always larger and shorter than wild-type cells, and their orientation in the filament was inverted. Consistent with increased peptidoglycan width and incorporation in the intercellular septa, filaments were longer in the mutants, suggesting a role for MreB, MreC, and MreD in the construction of septal peptidoglycan that could affect intercellular communication required for diazotrophic growth. IMPORTANCE Most studies on the determination of bacterial cell morphology have been conducted in heterotrophic organisms. Here, we present a study of how the availability of inorganic nitrogen and carbon sources influence cell size and morphology in the context of a phototrophic metabolism, as found in the multicellular cyanobacterium Anabaena. In Anabaena, the expression of the MreB, MreC, and MreD proteins, which influence cell size and length, are regulated by NtcA, a transcription factor that globally coordinates cellular responses to the C-to-N balance of the cells. Moreover, MreB, MreC, and MreD also influence septal peptidoglycan construction, thus affecting filament length and, possibly, intercellular molecular exchange that is required for diazotrophic growth. Thus, here we identified new roles for Mre proteins in relation to the phototrophic and multicellular character of a cyanobacterium, Anabaena.

Structural Determinants and Their Role in Cyanobacterial Morphogenesis

“…Although not further discussed in this review, cellular morphology can be changed by nutrient composition [ 58 ] as well as stresses such as temperature, salt, ultraviolet (UV) light and drought and are connected with the formation of ROS (reviewed by [ 59 ]). The aforementioned examples show the complexity and diversity of the environmental cues that influence morphology and cell shape in cyanobacteria.…”

“…A notable exception of the essential nature of mreB is Anabaena where mreB affects cell shape but was found to be dispensable for cell viability with combined nitrogen and did not affect chromosome segregation or placement [ 84 ]. Anabaena mreB , mreC and mreD deletion mutants were all characterized by an alteration of cell size, regardless of the growth conditions [ 58 ]. In wild type Anabaena, single cells are longer than they are wide (in respect to the trichome growth axis).…”

“…In the mreB , mreC and mreD mutants, however, cells became more coccoid and seemingly inverted their orientation within the trichome, being wider than long ( Figure 1 ). MreB, MreC and MreD additionally affected the Anabaena trichome length, possibly through a strengthening of the septal cell wall, which was found to be increased in diameter in the three mutants [ 58 ].…”

“…Given this vast regulatory network, mutations that affect processes such as cell wall synthesis [ 26 , 101 , 105 , 112 , 113 ], intercellular transport [ 114 , 115 ] and cell division [ 88 , 90 , 116 , 117 ] may alter cell shape. Consequently, the function of the mreB gene or the entire mreBCD operon [ 58 , 84 ] was analyzed in gene overexpression or gene deletion mutants of these aforementioned processes (reviewed by [ 59 ]). In all studied cases, upregulation of mreB is associated with the transition from coccoid to rod shaped cells.…”

“…PG biosynthesis enzymes, which are associated with the MreB-driven elongasome [ 68 , 124 ], were furthermore identified by several different groups to be essential for heterocyst formation [ 13 , 125 , 126 ], strengthening the importance of MreB function for heterocyst development. Additionally, mreB and mreC but not mreD are essential for diazotrophic growth of Anabaena , with a supposable function subsequent to heterocyst formation as Anabaena mreB , mreC and mreD mutants still differentiated heterocysts [ 58 ]. In the Anabaena wild type, cells are shorter during diazotrophic growth and longer in the presence of combined nitrogen [ 58 ].…”