lactose

Prokaryotic Gene Regulation

lactose or glucose as a sole carbon source, while certain mutant

wild-type Lac+ and those unable to use lactose L- and said that a

src="graphics/minus.xbm"> cells to be unable to grow on lactose. The

are grown in the presence of lactose or related molecules. In nature,

expressing the genes of the lac operon only in the presence of lactose

keeps the cell from wasting energy when lactose is absent. (This is

can be induced by adding the natural inducer, lactose, which causes a

lactose), causes a 1000-fold increase in expression. Because IPTG causes

operon. For example, lactose induces the genes of the lac operon but not

Able to grow on lactose as a sole carbon source. Wild-type

or lactose as a sole carbon source. To be Lac,

the lac Z gene, sometimes abbreviated b-gal.) and lactose permease (the

protein product of the lac Y gene) in the presence of lactose.

Beta-galactosidase is an enzyme that hydrolyses the sugar lactose (which is,

by itself, not metabolizable by E. coli) into glucose and galactose.

Lactose permease is a membrane-bound protein which allows lactose to pass

Unable to grow on lactose as a sole carbon source. Unless otherwise

the presence or absence of lactose.

expressed in the presence of lactose nor in its absence.

Coding region for the repressor proteinof the lactose operon.

(lac O, see below) in the absence of lactose. When bound, it blocks

the presence of lactose, lactose binds to the lac repressor protein,

to lactose. This type of mutation alone causes expression of lacZYA

lac operon operator DNA site. This is where the lactose

which prevents lactose repressor protein from recognizing and binding to

lactose. (see discussion of Lac)

Structural gene for lactose permease protein. It and the

on lactose. (see discussion of Lac)

coding region resulting in the absence of functional lactose permease. This

Structural gene for lactose trans-acetylase protein. This enzyme

is not required for growth with or with out lactose and is usually ignored

The b-gal levels with lactose are twice normal because there are two

of lactose, these cells will be Lac.

lactose will be wild-type. Since there is only one functional copy of

b-galactosidase, the b-gal levels in the presence of lactose are half of

expression and enzyme levels will be high with and without lactose.

Without lactose, only the lac operon on the F'-plasmid is expressed;

with lactose, both will be expressed.

lactose permease, so we can ignore it - keeping in mind that it does

MIT Biology Hypertextbook: Enzyme Mechanisms

lactose --> glucose + galactose

If you are lactose intolerant, you can buy lactase in a powdered form to

Membrane Transport Problem Answers

a mechanism by which galactose is transported into the intestinal

7.01Recombinant DNA Practice Problem

iv) Why can't you use lactose alone instead of glucose?

turn the lacZ+ cells blue. Lactose to induce the lac operon on the

iv) If you used lactose as the sole carbon source, the colonies

for growth on lactose as the sole carbon source.

2 Sugars

Sucrose and lactose are composed of two sugars and are therefore termed disaccharides.

Membrane Proteins Introduction

if an E. coli bacterium detects a high concentration of lactose in the

it begins synthesizing proteins to take in and metabolize the lactose. But,

Membrane Transport Mechanisms

Then those protons are coupled to lactose at the lactose permease

transmembrane protein. The lactose permease uses the energy of the proton

moving down its concentration gradient to transport lactose into the cell.

3 Enzyme Kinetics

lactose -----------------------> glucose + galactose

y-galactose -----------> y + galactose

in the 'normal' reaction, y-galactose = lactose = glucose-galactose (y=glucose)

ONPG = ONP-galactose (ONP = o-nitro-phenol)

X-gal = X-galactose (X = 4-chloro-3- bromo indole)

ONPG --------------> galactose + ONP

X-gal ---------------> galactose + 4-Cl-3-Br-indigo

3.2 Basic Kinetics

S substrate (lactose)

P product (glucose or galactose)