Breathtaking Bacteria Chapter 18 Part 2

I. Major Characteristics A. DNA one, double stranded, circular molecule tightly packed into a “nucleoid” region no membrane surrounding it Plasmids - smaller circular pieces of DNA outside nucleoid region

B. Reproduction Divide by binary fission , DNA replicates from a single origin of replication Asexual- no mating involved - Most offspring are genetically identical to parent Fast process - many can divide every 20 minutes Relatively high rate of mutation due to speed of reproduction Mutation rate helps bacterial colonies to survive better

C. Genetic Recombination While bacteria do not reproduce sexually, they are able to have some recombination of genes with other bacteria through one of the following methods:

1. Transformation Uptake of “naked” foreign DNA from the surrounding environment (like the S-strain/R-strain killing mice) Live, nonpathogenic cell takes up a piece of DNA that includes the allele to make it pathogenic Foreign allele is incorporated into the bacterial chromosome and replaces the original allele by crossing over Many bacteria have receptors on their surface proteins that aid in uptake of naked DNA only from closely related species Calcium - can be added to bacteria without these receptors (like e.coli) and will artificially stimulate the bacteria to take up naked DNA

2. Transduction phages carry bacterial genes from one host to another

Transduction (cont.) When a virus is reassembling in it’s host (end of lytic cycle), a small piece of bacterial DNA from host is accidentally packaged into capsid of virus Phage can attach to another host and inject the bacterial DNA into it Crossing over incorporates this DNA into the host cell RANDOM

3. Conjugation Direct transfer of genetic material between two bacterial cells that are temporarily joined. Bacterial version of “sex” One way transfer Donor = male - uses sex pili to attach to recipient = female Bridge forms between two bacteria and DNA can be transferred “Maleness” results from a special piece of DNA called F Factor.

D. Plasmids Small, circular, self-replicating DNA molecule separate from bacterial chromosome Can remain separate from chromosome Can become part of bacterial chromosome and replicate with it = episome No protein coats and can’t exist outside the cell Generally beneficial to bacteria Small number of genes that may help bacteria survive in stressful environments

1. F Plasmid Contains 25 genes required for production of sex pili F+ = cell that contains F plasmid - donators F- = cell without F plasmid - recipients Heritable F+ bacteria give rise to F+ offspring F+ x F-  F+ (male) replicates it’s DNA, transfers copy to F- (female) converting it to F+ (male). Now the newly converted F+ bacteria can make sex pili and transfer its plasmid to new bacterial cells

2. Conjugation and Recombination in E. coli Hfr cell-F factor becomes part of bacterial chromosome. During conjugation - F factor replicates and gets transferred to F-, but some of the bacterial chromosome can be taken with it. Recipient cell is temporarily “diploid” for some genes and crossing over can occur When bacteria divides, it has the new genes Any pieces of Hfr DNA left will be degraded

3. R-plasmids and Antibiotic Resistance R-plasmids-“Resistance” plasmids - contain genes that code for resistance to antibiotics When a bacterial population is exposed to antibiotics, any “sensitive” bacteria are killed by the antibiotics Bacteria that contain the R plasmid with resistance to the antibiotic survive These bacteria then reproduce, increasing the number of antibiotic resistant bacteria (natural selection) R-plasmids can also be transferred during conjugation and may carry as many as 10 genes for resistance to different antibiotics

E. Transposons Piece of DNA that can move from one location to another in a cell’s genome Never exist independently Recombination between transposon and a “target site” that comes in contact with the transposon

E. Transposons Can move within a chromosomes, from a plasmid to the bacterial chromosome, or from one plasmid to another (multiple genes for antibiotic resistance are in single R plasmid due to transposition) Can jump from one area to another, OR - can replicate first and the copy inserts somewhere else Genes can be moved anywhere (not like other processes where crossing over had to occur)

II. Control of Gene Expression in Bacteria A. Purpose helps individual bacteria cope with changes in their surroundings ex. E. coli in human intestine - rely on what the host eats for it’s nutrition. Bacteria must be able to turn genes on and off depending on its nutritional needs

B. Metabolic control Regulates which metabolic pathways are turned on and off. Done by one of the following 2 methods:

B. Metabolic control 1. Regulation of enzyme activity ex. Feedback inhibition - when the end-product starts to accumulate, it will inhibit one or more of the enzymes in the pathway turning it off

B. Metabolic control 2. Regulation of gene expression - control of enzyme/protein production by controlling transcription and translation (turning genes on and off). In bacteria - this regulation occurs with Operons

C. Operon Contains all the necessary components for controlling a metabolic pathway including: 1. Operator - on/off switch located in the promoter region of the DNA (before the genes) 2. Promoter - binding site for RNA polymerase 3. Transcription unit - all of the genes necessary for a certain metabolic pathway

C. Operon 4. Regulatory gene - occurs somewhere else in the DNA and codes for a repressor (off switch) 5. Repressor – a protein that switches an operon off. Specific to one operon 6. Corepressor - binds to repressor and activates it, changes it’s shape so it can bind to the operator and turn the pathway off

Negative Control…The Repressible Operon This pathway is called repressible because the system is normally on, but can be turned off when there is enough resources available for the bacteria. Ex. Trp operon synthesizes tryptophan which is vital for bacterial survival.

D. Negative Control…The Repressible Operon 1. Normally the operon is in the on position, one long mRNA is made for the 5 enzymes required in the pathway. The mRNA will attach to a ribosome, produces the enzymes and the enzymes will make tryptophan

2. Host eats Thanksgiving dinner Turkey contains lots of tryptophan Why would the bacteria use its resources/energy to make tryptophan when it can get it from its host? - it doesn’t

Trp operon (cont) 3. Tryptophan is a corepressor in this pathway. It will bind to the repressor molecules (floating in cytoplasm but inactive) When trp binds to repressor, shape of repressor changes and it becomes active Active repressor then binds to operator region of DNA and turns the operon OFF No mRNA is made and the enzymes to make trp are no longer produces

4. After that great turkey dinner is digested by the host and the tryptophan levels decrease again, the repressor becomes inactive again, releases from the operator. The bacteria need to make their own tryptophan now The operon is turned back on

Negative Control-- The Inducible Operon In this pathway, the reprossor is made in it’s active form and automatically binds to the operator. The system is normally OFF but can be turned on (induced) when it is needed. Ex. The lac operon produces three enzymes to break down lactose (milk sugar) for energy.

Lac operon (cont) 1. Host drinks a big glass of milk (contains lots of lactose plus some allolactose) 2. Allolactose binds to the repressor and releases it from the operator region 3. RNA polymerase can then go on and transcribe the mRNA needed to make the enzymes for breaking down lactose

F. Positive Control of Gene Expression There is another factor controlling this operon Glucose is the best energy source for the bacteria. Lactose doesn’t provide as much ATP as glucose does.

Positive Control of Gene Expression Enzymes to break down lactose will only be made if: a. lactose is present (see process above) b. glucose levels are low

Positive Control of Gene Expression 1. cAMP accumulates cAMP binds to a regulatory protein called cAMP receptor protein (CRP) 3. This complex (cAMP+CRP) is an activator of transcription and binds near the promoter of the operon and makes transcription faster by making it easier for RNA polymerase to bind to promoter 4. If glucose levels build up, cAMP/CRP will release and transcription will slow down