Anabaena is a simple multicellular photosynthetic cyanobacterium. In the absence of fixed nitrogen, certain newly developing cells along a filament express genes that code for nitrogen-fixing enzymes and become non-photosynthetic heterocysts. The specialization is advantageous because some nitrogen-fixing enzymes function best in the absence of oxygen. Heterocysts do not carry out photosynthesis but instead provide adjacent cells with fixed nitrogen, in exchange receiving fixed carbon and reduced energy carriers.

As shown in the diagram above, when there is low fixed nitrogen in the environment, an increase in the concentration of free calcium ions and 2-oxyglutarate stimulates the expression of genes that produce two transcription factors (NtcA and HetR) that promote the expression of genes responsible for heterocyst development. HetR also causes production of a signal, PatS, that prevents adjacent cells from developing as heterocysts.

Based on your understanding of the ways in which signal transmission mediates cell function, which of the following predictions is most consistent with the information given above?

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In an environment with low fixed nitrogen, free calcium concentration increases and leads to an increase in NtcA and HetR. If a calcium-binding compound is added, the free calcium concentration would not increase since it would bind with the compound. This leads to reduced NtcA and HetR production, which subsequently results in reduced Heterocyst development.

If patS is overexpressed, there should be increased inhibition of heterocyst development, meaning less heterocysts.

When nitrogen is abundant, free calcium would not be high.

Loss of hetR means less heterocyst development.