Final answer:
Heterocysts are specialized cells in some cyanobacteria like Anabaena that fix atmospheric nitrogen. They form in response to low nitrogen environments and are regulated by the transcription factors NtcA and HetR, as well as communication signals like PatS. The loss of hetR or a mutation in NtcA would have a significant impact on the development of these specialized cells.
Step-by-step explanation:
A heterocyst is a specialized cell found in some filamentous cyanobacteria such as Anabaena. Its main function is to fix nitrogen from the atmosphere, converting it into ammonia (NH3) that can be utilized by the plant for growth. This is critically important in environments where there is low fixed nitrogen. Heterocysts are adapted for nitrogen fixation because they are able to create a micro-anaerobic environment that is necessary for the function of the nitrogenase enzyme, which is oxygen-sensitive. In addition, heterocysts form a mutualistic relationship with vegetative cells, where they provide fixed nitrogen to these cells, and in return, receive fixed carbon and reduced energy carriers.
Heterocyst formation is regulated by cellular communication and gene expression changes triggered by environmental conditions. When nitrogen is scarce, an increase in cellular calcium ions and 2-oxyglutarate promotes the expression of two transcription factors, NtcA and HetR. HetR also synthesizes PatS, a signaling molecule that prevents neighboring cells from developing into further heterocysts, thus maintaining a regular pattern of heterocysts along the filament. This ensures an efficient distribution of nitrogen-fixing cells that can support the entire colony.
It is therefore inaccurate to suggest that loss of the hetR gene would induce heterocyst development (prediction D), since HetR is crucial for heterocyst differentiation. Likewise, overexpressing the patS gene would not cause the formation of more heterocysts; instead, it could suppress heterocyst development (prediction B). Lastly, a mutation making NtcA nonfunctional would likely disrupt the communication and differentiation pathway, allowing neighboring cells to form heterocysts (prediction D)