Hello Amanda,
The quick answer to your question is that beta-galactosidase, which is encoded by the lacZ gene of the lac operon, is an enzyme involved in the metabolism of lactose, a sugar present in milk. More specifically, beta-galactosidase is best known for its ability to cleave lactose (a disaccharide) into two monosaccharides: glucose and galactose. In an alternative reaction, beta-galactosidase is also involved in the conversion of lactose into its isomer allolactose. In E. coli cells, a small amount of lactose also spontaneously isomerizes into allolactose. Now, let’s step back and briefly review some of the key features of the lac operon.

As you likely know, genes in prokaryotic cells are often organized in clusters called operons — in this way, they can be transcribed together using the same promoter. Similarly, the expression of the genes within an operon is co-regulated. As you can see, operons are a great example of cellular multitasking! The lac operon is one of the most well-studied operons in E. coli, and its cluster of three genes — including lacZ, lacY, and lacA — encode proteins involved in lactose metabolism. The lacY gene encodes a permease that functions as a membrane-spanning transport protein to bring lactose into the cell, and the lacA gene encodes a transacetylase enzyme that transfers an acetyl group from acetyl CoA to beta-galactosidase.

The three genes that comprise the lac operon are not constitutively expressed; instead, they are expressed in a highly regulated manner only when cells need them (i.e., when lactose is the available sugar source). In the absence of lactose, a repressor protein called lacI binds to the operator region of the lac operon and blocks RNA polymerase from binding to the upstream promoter, effectively inhibiting the transcription of the downstream lacZ, lacY, and lacA genes. As a result, cells produce very low levels of beta-galactosidase, permease, and transacetlyase enzymes in the absence of lactose. When lactose is present in the environment, its isomer allolactose binds to and induces a conformational change in the lacI repressor protein, inhibiting its ability to bind to the lac operator region; this allows RNA polymerase to effectively bind to the lac promoter and transcribe the downstream lacZ, lacY, and lacA genes.

In order to help you understand how this happens, we’ve provided a collection of useful links to Scitable articles and other resources below. You might be interested to learn about how the lac operon is regulated when both lactose and glucose are present in the environment. We hope you enjoy reading more about these impressive examples of coordinated gene expression and regulation!

Check out these articles to learn more about beta-galactosidase and the lac operon:

http://www.nature.com/scitable/topicpage/positive-transcription-control-the-glucose-effect-1009

http://www.nature.com/scitable/topicpage/negative-transcription-regulation-in-prokaryotes-1013

http://www.nature.com/scitable/content/the-lactose-operon-of-escherichia-coli-7005

http://www.nature.com/scitable/topicpage/operons-and-prokaryotic-gene-regulation-992

http://faculty.clintoncc.suny.edu/faculty/michael.gregory/files/Bio%20101/Bio%20101%20Lectures/Gene%20Regulation/gene.htm

And here are some links to helpful animations that nicely illustrate how the lac operon is regulated:

http://glencoe.mcgraw-hill.com/sites/9834092339/student_view0/chapter15/the_lac_operon.html

http://bcs.whfreeman.com/thelifewire/content/chp13/1302001.html

Hello Ananda,
You’ll be happy to hear that you could most certainly enter a doctoral program in an area focused on cancer genetics after you complete your M.S. in Genetics and Plant Breeding! It’s not at all uncommon for people to change course in their careers, and the experience you’ve gained through your Master’s program will no doubt be very helpful in your doctoral program. We have provided some useful links below to help you identify cancer biology graduate programs that may be of interest to you. In addition to the school rankings, location, and programs offered, it is important to consider the type of research you might be interested in carrying out and to look for schools that have options for research labs in that area. To do that, it is important to check out the research interests of professors at the schools you’re considering. We wish you the best of luck as you complete your M.S. and begin the application process for doctoral programs. It is certainly an exciting time for you!

To help facilitate your search for Ph.D. programs in cancer biology, please explore the wealth of information provided in the following links:

http://www.usnews.com/rankings

http://grad-schools.usnews.rankingsandreviews.com/best-graduate-schools/top-science-schools

http://www.topuniversities.com/university-rankings/world-university-rankings/2010/results

http://www.arwu.org/index.jsp

http://chronicle.com/article/NRC-Rankings-Overview-/124733/

http://nces.ed.gov/collegenavigator/

http://www.petersons.com/graduate-schools.aspx

Thank you so much! That was very helpful

Hello Julius,
As you likely know, receptors are critical for cell communication. Cell communication is largely dependent on extracellular signaling molecules, which are produced by cells to transmit signals to neighboring cells (paracrine signaling), to distant cells (endocrine signaling), and to themselves (autocrine signaling). Examples of signaling molecules include proteins, small peptides, amino acids, nucleotides, steroids, retinoids, fatty acid derivatives, and dissolved gasses (e.g., nitric oxide, carbon monoxide).

Cell communication depends on a complex system of proteins — including cell surface receptors that interact with signaling molecules and a variety of intracellular signaling proteins — that collaborate with each other to form a signaling pathway. Ultimately, activation of signaling pathways results in the alteration of target proteins, which can change cell behavior. Some of the key players in signaling pathways include gene regulatory proteins, ion channels, components of metabolic pathways, and components of the cytoskeleton. Many signaling molecules cannot readily cross the plasma membrane, and they must bind to cell surface receptors to transmit their signals. However, some small signaling molecules can diffuse across the cell membrane and bind to receptors inside the cell (i.e., intracellular receptors), either in the cytosol or in the nucleus (i.e., nuclear receptors).

So, what experiments might you carry out to provide evidence supporting the existence of a receptor? Before we consider some specific experiments, let’s ponder what you might already know about that receptor. Does the receptor have a homolog (e.g., in the budding yeast Saccharomyces cerevisiae)? Do you want to isolate the corresponding receptor in mammalian cells? Many receptors were first identified in simple organisms like yeast, and the corresponding genes were later cloned in mammalian cells. Oftentimes, this was accomplished using the polymerase chain reaction (PCR) technique together with degenerative sets of PCR primers whose DNA sequence was based on the known DNA sequence of the corresponding yeast receptor.

After cloning a mammalian version (i.e., homolog) of a yeast receptor, would you want to know when and where it is expressed in mammalian cells? With the DNA sequence of the mammalian gene in hand, you could clone a fragment of it into an expression vector, which could in turn be used to produce the corresponding protein fragment in bacterial cells. Once purified, the bacterially expressed protein (i.e., recombinant protein) could be used to generate antibodies that specifically recognize and bind to the receptor in mammalian cells. By labeling the receptor antibodies either directly or indirectly with a fluorescent tag, you would be able to address many questions, including where the protein localizes, what other proteins it interacts with, what functions it carries out, and how its expression is regulated.

We hope this introduction has provided you with a starting point as you begin considering additional experiments you might design to study receptor function and cell signaling.

To learn more about receptors, check out these links:

http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=mcb&part=A5717#A5720

http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=mcb&part=A2626#A2658

http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=mcb&part=A5756#A5762

To learn more about PCR, follow these links:

http://www.nature.com/scitable/definition/polymerase-chain-reaction-pcr-110

http://www.nature.com/scitable/topicpage/scientists-can-make-copies-of-a-gene-6525968

To learn more about systems for using bacterial host cells to produce a protein of interest, see these links:

http://www.nature.com/scitable/topicpage/recombinant-dna-technology-and-transgenic-animals-34513

http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=mcb&part=A1706#A1708

To learn more about methods for studying the function of specific receptors, check out these links:

http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=mcb&part=A1965

http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=mboc4&part=A1363#A1399