A sense of taste is essential for all living organisms to be able to discriminate between palatable and unpalatable material. In humans there are five basic tastes ------ sweet, bitter, salty, sour and umami. The way in which the tastes sweet and bitter are recognized has not been explored at a molecular level until recently. Most researchers postulated that there might be receptor molecules that bind and recognize tastes and several genes that may be responsible for this have been discovered.

However, direct evidence that they are functioning as taste receptors was lacking. For many years we have been using the fruit fly, Drosophila melanogaster, as a useful experimental animal to find out which genes are responsible for the ability to taste sweet foods. Drosophila's genes have been extensively studied since 1910 and the work of T.H. Morgan. We are now able to isolate flies with mutations that have led to behavioral or neurological defects.

Flies like sweet food. They sense sugar by using bristles located on the legs and the mouthparts, the labellum. Many years ago we found a gene responsible for controlling the taste sensitivity to a particular kind of sugar in Drosophila. Now we have cloned the gene and showed that the gene encodes a new protein that is embedded in a membrane which functions as a taste sensor. This is the first piece of research in which the function of a taste receptor has been proved in an organism.

We found that the taste sensitivity to trehalose differs among some laboratory strains of Drosophila. Trehalose is a disaccharide composed of two glucose molecules and this sugar is found in natural products such as yeast and mushrooms. To determine the taste sensitivity of flies to sugar, we developed a behavioral test that we called the two-choice preference test. Two kinds of sugar-agar solutions, each colored with either a blue or a red food coloring were placed on a microtest plate as shown in Photo (A). Flies are able to detect the difference of sweetness between the two sugar solutions and select the sweeter sugar. The sucrose solution was colored red and the trehalose solution was colored blue. At a certain concentration of each sugar normal flies were colored blue, while the flies with a low-sensitivity mutation were colored red. The color of the sugar solution ingested can be easily observed under a dissecting microscope as shown in Photo (B). In this way we could easily measure the taste sensitivity of the flies. We have located the responsible gene, Tre, on the chromosome map. Further genetic studies suggested that Tre may be the gene for the trehalose receptor. Since then we have attempted to clone the Tre gene. Four years ago, we were excited to find a DNA fragment that may be the part of the Tre gene. This finding was made through a molecular study using a mutation called poxn. In flies with the poxn mutation all of the taste bristles on the legs and the mouthparts are genetically transformed into simple bristles that sense only physical stimuli. This mutation provides us with a way to screen out the genes that are responsible for the sense of taste. By this strategy we have identified a putative gene for Tre and revealed the structure of the protein in the membrane. We then deleted this gene in the fly and found that taste sensitivity to trehalose was lowered. In turn we introduced the normal gene into the mutant fly and showed the low-sensitivity character was restored. We also showed that this gene is expressed in the taste receptor cells.

On the same day our paper was accepted by Science the whole genome sequence and the annotated genes of Drosophila was published. Our speculated structure of the taste receptor gene was determined to be correct. We were very happy that we were able to accomplish this achievement from the isolation of a taste mutant and the cloning of the gene in Japan. Our study suggests that there should be additional receptors for other sugars. We are now starting to unravel the machinery of sweet taste reception at a molecular level.

Teiichi Tanimura is in the Department of Biology, Graduate School of Sciences. He graduated from Tohoku University and then took a position at the National Institute for Basic Biology and Fukuoka University. He is studying on the molecular and behavioral genetics in Drosophila.