Lambda Phage Lytic vs Lysogenic Decision Explained (Microbiology)

[00:00:06]hello everyone and welcome to the channel so today we're going to be talking about the lambda phage and its decision making process so we're going to be looking at lytic and lysogenic growth so let's just dive right in we're going to start with this image here so essentially what we're looking at is infection of a bacteriophage into a host cell and upon this infection we can see that there are two outcomes that we observe and those are lytic growth and lysogenic growth so if you don't already know what those are i'll just go through them right now lytic growth is essentially where the bacteriophage exploits the hosts machineries and resources to multiply itself and create more bacteriophages and after that the cell the host cell is ruptured where all these new bacteriophages that have been created are released and they can go on to infect more host cells so that is lytic growth and lysogenic growth as you can see here is a little bit different and this is where the host genome is being basically integrated with this phage genome which is known as the prophage and as you can see there are little red dots that are called lambda repressors and we will get into that in a second but this is general overview of what's going on here so let's just jump on into some of the new information so we're going to start here this is a very general picture to give you context essentially we're looking at a circular genome we see all these different promoters all these different um jeans that we're gonna express and we're gonna get into all of this this looks very muddy right now but hopefully by the end of this video a lot of these things will make sense so today i i want to start with these three promoters actually so we know we're looking at either lytic growth or lysogenic growth right those are our two options that we can go into so there's different promoters that will start to express different proteins that will put us into those scenarios right so we have something called pr and pl so leftward promoter pl rightward promoter pr the names are pretty self-explanatory and these express their respective proteins so pr if you look at this image pr as you can see expresses right word or downstream and it's expressing crow something called crow which we'll get into in a bit and our pl is expressing leftward and it expresses something called n which isn't shown here and n is known as anti-termination so essentially what anti-termination is is that when we have a polymerase that is transcribing it will normally stop when it is indicated to do so right it stops at a stop signal but when we have anti-termination what that means is it's going to skip the stop signal and go further so you end up with a longer transcript now if we look at prm this is in the middle in between pr and pl and it expresses leftward as well and it expresses something called ci ci is also known as lambda the lambda repressor the names those two names are interchangeable so if i say ci or lambda i mean the same thing so one thing i want you to just i just want to lay some groundwork with you here first pr and pl i want you to associate these two promoters with lytic growth okay if we have pr and pl working we're in lytic growth and if we have prm working here we're dealing with lysogenic growth okay so another thing i want just a little side detail which we will come back to this isn't absolutely crucial in this point in time but pr and pl are very strong promoters and that will come back in a second we can just put this information in our back pocket for now but pr and pl are strong and prm is a weak promoter and that will come back to full circle it'll help explain some things so let's just take the basics that we've established and go one step further with it right so we know we have these promoters p r p l p r m and they express their respective protein crow nci also known as lambda okay something that we're going to add here just another layer is that these promoters have operator sequences so there are three operator sequences as you can see in this image or1 or2 and or3 and depending on what promoter we have expressing right depending on if we're in lytic or lysogenic growth so depending on if we have pr and pl or prm expressing their proteins we're going to see binding from these proteins at these operator sequences at different affinities so that is a very loaded sentence let's just break that down really quick here so let's take our scenario if we have pr expressing crow right pr expressing rightward to make crow crow as you can see that i've listed crow binds to or3 with the highest affinity or3 is the highest affinity as opposed to or2 and one but those two are relatively the same affinity okay crow binds to or3 with the highest conversely if we have prm expressing ci so expressing leftward making ci ci it binds to or1 with the highest affinity so it's basically the opposite of crow what it does here the binding sites i should say it binds at the opposite end with highest affinity so what that does we'll get into that in a second but i want you to first notice from this image that our promoters are you know conveniently overlapping with these operator sequences and that does something here so our pr as you can see it overlaps with or1 and our prm overlaps with or3 okay so let's just break this down now if we have pr expressing crow and crow binds with highest affinity at or3 like i mentioned it is essentially in the way of prm it's binding where prm is located it is blocking it basically right so when we have our rna polymerase once that gets introduced here it's not going to go at prm because the prm is completely blocked off by crow right now so where's the only place it can go the other promoter pr right because it's open so it's going to bind there and it's going to express rightward and we're just going to make more crow and we're going to stay in the lytic cycle that's basically what's happening and that means crow did its job it wants to stay lytic right so if we look at the other side of the coin prm expressing ci ci would be binding at or1 with the highest affinity and by doing this it is over or1 overlaps with pr as you can see and because it's binding at or1 because our lambda repressor or ci is binding there it is blocking essentially pr it's in its way and rna polymerase will not go there it can't because it's completely blocked off so it's going to go at the other promoter that's available there prm and then it's going to do that and it's go it's going to express leftward to make more ci or more lambda repressor and it's going to keep it in lysogenic growth okay so i know that sounds like a lot of information i will show you a couple of visuals here in a second and hopefully that will help you digest what i just said so this is the first image that i want to show you as you can see this is lytic growth this is what little growth looks like we have crow being expressed crow we said binds at or 3 with very high affinity and it is blocking prm right and then as you can see our rna polymerase it goes to pr that's where it's going and it's going to go express rightward where crow is so that's the direction we're going in and we are staying in lytic growth that's what that looks like and if we look here i just want you to direct your attention to the bottom there that is what lysogenic growth looks like this is not as labeled as my other image but i will still go through this with you we have prm making ci right we're expressing ci ci is binding at or1 with the highest affinity it's not labeled but that's where it is and it's also binding at or2 cooperatively and as you can see there's a bunch of these and they are tetramerizing so i want you to look on the right side here this is just like almost like a little bit of an anatomy of the lambda repressor of ci there's a dna binding site an activating region a tetramerization region and if you overlay this image on the right with how they're positioned on the bottom there in the red image you can see that they're attaching to the dna with the dna binding region they are activating one another and tetramerizing or interacting with their tetramerization region so you can see their their anatomy is almost fully aligned with one another right and the very closest lambda to the rna polymerase is actually interacting with it as you can see they are touching there it is activating it so rna polymerase is going to end up going leftward from prm and we're going to keep getting ci so here we see that ci tetramerizes and if you go back to my other image there crow does not look the same as lambda okay lambda almost looks like a little dumbbell crow is just a little dimer crow stays as a dimer it dimerizes and ci or lambda tetramerizes as you can see so that's what i want you to distinguish between those two and if you look at the top here we see a couple of new things that might cloud your knowledge with c2 and pre and c2 binding site and that looks very confusing but we will get into it in a second so this is just a close-up of that previous image there so let's talk about c2 and pre so we know we have pr prm pl and they do their thing that we've just established this whole time right now if we take our story from the top if we have infection from a bacteriophage to a host cell right i want you to remember that i said pr and pl are very strong promoters and prm is a weak promoter right and that's where this comes in upon infection we are immediately right off the bat gonna start with lytic growth because pr and pl are such strong promoters right off the bat those are going to express their proteins so boom pr is expressing crow as you can see at the top right and not only are we expressing crow we see we're going further than that we're passing pre we're expressing c2 as well so this little collection here on the right pre c2 c2 binding site these things are these are very unstable things that we're dealing with now we're entering into very unstable territory so c2 it needs to bind to c2 binding site it has a full on binding site for it because it's pretty unstable and it's also very close to our pre so another promoter in the middle here as you can see in this middle picture pre is expressing leftward and we'll get into that in a second but it needs c2 to bind at the c2 binding site first um and this is because pre is an extremely weak promoter okay so because it's so weak it needs c2 to bind at the c2 binding site and this is basically what's going to help position rna polymerase at pre so that it can properly go express leftward and transcribe whatever it's going to do in a second that i will get into does that make sense i'm hoping this is making sense with this image here and that you're following along so it's very unstable it's binds to the c2 binding site c2 is unstable binding there that's where it's going to position rna polymerase because pre is also very weak and so now we see expression leftwards right we see it passing crow but we're not making crow we're actually making antisense crow here right because we're going in the opposite direction and we're also passing along ci so we're making ci we're making our lambda repressor right and if we have enough ci being made remember what does ci do ci if we follow along to the bottom image now ci like we said binds to or1 with super high affinity right it's going to bind to or1 slash or2 cooperatively and therefore it's blocking pr right that's what we said it does it's doing its job and so because pr is now being kind of blocked this is where our rna polymerase is going to position at prm instead and it's going to make more c we're going to we're gonna start transcribing leftward so we're getting more ci and we're in lysogenic growth now right so this is kind of where the switch happens if we're making crow we're in the lytic cycle as we said at the top right off the bat upon infection you're usually gonna get expression of crow because pr is so strong and pl which isn't shown in this picture but because they're such strong promoters we're starting off in the lytic but the switch happens because of c2 essentially our c2 if it's it's very unstable and unreliable but if it is binding to the binding site and positioning polymerase at pre we're going to start making ci and if we're making enough ci like i said it's gonna block prm because of the whole process we just went through it's gonna block prm because it binds there with very high affinity at or1 and now we're in lysogenic growth we're making ci now so that's basically the gist of it that's kind of how it goes this image is a really good summary if you can kind of almost tell the story to yourself following along with this image then you definitely have a good grasp okay so we're back with our circular genome that we started with so as you can see at the top here these all these promoters p r p r m p r e we've just broken them down a bit right all these little sequences here n crow all that stuff they make a lot more sense now right another thing that i want you to start looking at here because there's obviously other things on this genome i want you to direct your attention to the left side where you can see something called excisionase and integrase and p1 which is their respective promoter that's it basically these are associated with like the lysogenic cycle these um genes these sequences right and this is because i just want you to logic this out really quick we said lysogenic growth is where we have integration of the phage genome into the host genome right that's where it stays there and it's known as the prophage so in order to do that we need proteins like integrates and excisionase that's what that is useful for so naturally they're associated with the lysogenic cycle so this left side here we're gonna associate this left side with the lysogenic cycle and if we move on to the right we see q we see labeling that says lysis proteins head genes tail genes this is all lytic cycle stuff okay because like i said the lytic cycle we said it's associated with it concerns itself with exploiting the host and making more phages right we're making new phages so we need head genes and we need tail genes because we need to make new phage heads and tails because we're making more we're duplicating we're multiplying so obviously the right side here is going to be associated with the linux cycle so this is essentially we've covered the whole genome here and we kind of have an idea of what does what um so that is basically all that we have now so that's all for today i hope that that explained everything that you need for your exams on this topic be sure to like this video and to comment below if you have any other questions relating to the lambda phage or if you have any other video topics that you would like brain boost to cover and we will cook that up for you in 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