This video, we're going to further emphasize the importance of prokaryotic sigma factors. Recall in our previous lesson videos when we talked about the initiation of transcription, we mentioned that sigma factors are important for prokaryotes to initiate transcription. Here we're recalling from our previous lesson videos that prokaryotic transcription initiation requires a sigma factor molecule. The sigma factor molecule is going to bind to the promoter sequence and help to recruit the RNA polymerase so that the RNA polymerase can bind and initiate transcription. Now, cells can actually have many different sigma factors, and these different sigma factors can recognize different promoters on the DNA. Different sigma factors are needed to express different genes within prokaryotes. Standard sigma factors are used for expressing genes during routine growth. Standard sigma factors are almost always going to be expressed because they're always needed for routine growth. However, alternative sigma factors are going to recognize different promoters and control the expression of alternative gene groups. These alternative gene groups are not routinely expressed and are only going to be needed in very specific, certain scenarios. These alternative sigma factors will need to be present in order for these alternative gene groups to be expressed. For example, alternative sigma factors control the expression of specific gene groups during heat shock in bacterial cells. That's what we're showing you over here on the left-hand side as an example of these alternative sigma factors.
Again, these sigma factors are only present in prokaryotes, and they are not used by eukaryotes. Over here on the top, what we have is a prokaryotic cell, and notice it's saying it isn't that hot, I'll be fine. This prokaryotic cell is just in its normal temperature conditions. However, if we heat shock this bacterial cell here, by applying lots of heat in a short period of time, then, this bacterial cell will need to rely on heat shock proteins, and those heat shock proteins will need to be expressed. The only way those heat shock proteins can be expressed is if the appropriate sigma factors are available. If the sigma factor is present, like what we see over here on the left-hand side, then transcription of the heat shock genes will occur, and those heat shock proteins will protect the bacterial cell from high temperatures. Notice on the left-hand side over here, we're showing you the bacterial cell surviving the heat shock, and it's saying the heat never bothered me anyway. Now on the right-hand side, what we're showing you is what happens if the sigma factor is not present. If the sigma factor is absent and not available then no transcription of the heat shock genes will occur. If there are no heat shock proteins made then that is ultimately going to lead to the death of the bacterial cell. These prokaryotic sigma factors must be present in order for these specific genes to be expressed at the right time when they are needed.
Over here on the right-hand side, we're just showing you some examples of gene groups that are regulated by alternative sigma factors. Not that you need to memorize this list here, but just showing you a few different examples. For example, heat shock, like what we just talked about, relies on the presence of very specific alternative sigma factors. The stationary phase of survival, during a normal growth curve for bacteria, nitrogen assimilation, flagellar synthesis, misfolded protein response, and ion transport and uptake are all examples of some processes that require and rely on alternative sigma factors. The biggest takeaway from this video is that prokaryotic organisms rely on sigma factor molecules to initiate transcription during very specific situations for specific genes to be expressed. This here concludes our brief lesson on prokaryotic sigma factors, and we'll be able to apply these concepts as we move forward and learn more about prokaryotic gene expression as well. So I'll see you all in our next video.