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Small Genetic Changes with Major Consequences Host Change Alters Public Understanding of Life Sciences / Chemical Ecology PULS/CE 17 Newsletter April 2011 Host Change Alters Toxic Cocktail Due to a gene modification leaf beetle larvae attacking birch trees produce toxic cocktails that differ from the ones produced by conspecifics living on willows … p. 3 Small Genetic Changes with Major Consequences Small changes have big consequences, some of which may lead to the development of new weapons in the evolutionary arms race between plants and herbivores … p. 4 Antibiotics in situ Scientists are looking for antibiotics produced by microbial symbionts of leaf-cutting ants and study their function in the ecosystem … p. 5 PULS/CE 17 2 Newsletter April 2011 | Editorial PULS/CE 17 reports on two particular stories in- volving small molecular differences which have important consequences. A leaf beetle species has been shown capable of turning to birch trees instead of willows as hosts because of a minor “malfunction” in processing genetic information. A second example demonstrates that today’s Brassicaceae species can defend themselves successfully against herbivory by producing cer- tain glucosinolates after an amino-acid-produc- Amino acid sequence comparison of salicyl alcohol oxidases in different leaf beetles: a vacancy is ing enzyme switches from producing leucine to visible in the amino acid sequence of Chrysomela lapponica beetles feeding on birch (red area) . synthesizing glucosinolates (their strong taste More information on this topic is available on page 3. Picture: PNAS and smell is typical of cabbage) just because its polypeptide chain is shortened. Although Alice Schwarzer and molecular biology Little Differences seem to have nothing in common, you should be reminded that males and females have only one tiny genetic difference – tiny as far as the basic Dear Readers! course of development for a human ovum after fertilization is concerned: Only the composition of “The little difference and its huge consequences” the 23rd chromosome pair in Homo sapiens deter- was the title of one of the most successful books mines the developing embryo’s sex. This little XX to come out of the young Bonn republic. Alice versus XY difference has had huge consequenc- Schwarzer‘s ground-breaking work was published es, sociological and biological. We are a long way in 1975, when West Germany was democratically from understanding all of them. maturing and – despite its 26 years of peaceful existence – still somehow unsettled. Although Schwarzer had something completely Enjoy a beautiful spring and our new PULS/CE different in mind than the topics in this month‘s issue! newsletter – she was fighting for equal rights between men and women – “little differences can have major consequences“ is true in many contexts. Jan-W. Kellmann 3 PULS/CE 17 Research Highlight | Newsletter April 2011 means that birch feeders lack the enzyme salicyl Host Change alcohol oxidase (SAO), which is responsible for the oxidation from alcohol to aldehyde,” explains Alters Toxic Roy Kirsch, PhD student in the Department of Bio- Cocktail organic Chemistry and author of the study. Biochemical analyses revealed that the gland secretions of salicylaldehyde-producing willow Leaf beetles fascinate us because of their amaz- beetles contain this enzyme in strikingly large ing variety of shapes and rich coloring. Their amounts. The scientists labeled it SAO-W (W: larvae, however, are dangerous plant pests. willow). Using corresponding DNA sequence Beetle larvae are part of a food chain; they are data, they isolated and characterized the SAO- A leaf beetle larva (Chrysomela attacked by predatory insects and parasites. To B (B: birch) encoding gene from birch feeders. lapponica) emits toxic secretions, protect themselves, they emit substances from They found that the amino acid sequences of visible as vesicles, from its defensive their defensive glands. These defensive secre- both enzymes are 97 percent identical. However, glands as a chemical protection against tions contain toxins, such as butyric acid esters or SAO-B is inactive because 27 amino acids at the predators. Photo: Kerstin Ploss, MPI-CE salicylaldehyde, which the larvae sequester from beginning of the polypeptide chain are missing. chemical precursors they have ingested with their Further studies on the defensive glands of birch food. Most leaf beetle species use only one plant feeders have shown that the amount of messen- species for feeding and reproducing on. Larvae of ger RNA (mRNA) in the SAO-B gene was reduced the leaf beetle Chrysomela lapponica attack two by 1000-fold compared to that in willow beetles; different tree species: willow and birch. On the the protein and its enzyme activity were below one hand, the uptake of special plant molecules the detection level. The lack of enzyme activity is as substrates for toxin-producing enzymes is eco- caused by a mutation in the SAO-B gene located nomical for the beetle larvae; on the other hand, near the second exon/intron junction. The muta- the leaf beetles become strongly dependent on tion is responsible for changes in mRNA process- the host plant and its specific metabolites. Wil- ing, so-called alternative splicing, which leads to lows of the Salicaceae family have up to 5 per- the loss of 27 amino acids in the SAO-B enzyme. cent glycosylated salicyl alcohol (salicin) in their leaves, whereas birch trees contain none of these The scientists conclude that Chrysomela lappon- Roy Kirsch Photo: MPI-CE compounds. Hence, researchers have investigat- ica likely used willows exclusively as host plants ed how Chrysomela lapponica leaf beetles adapt- and later shifted to birch trees as well. “It is still ed to both birch and willow as host trees and unclear whether the gene mutation enabled the analyzed whether the loss of salicylaldehyde in host plant shift from willow to birch or whether it Original Publication: birch feeders is due only to the fact that the pre- was adapted in the course of evolution after the Kirsch, R., Vogel, H., Muck, A., cursor salicin is not available in birch. To test their shift to birch had occurred,” says Wilhelm Boland Reichwald, K., Pasteels, J. M., Boland, hypothesis, they offered willow leaves to hungry By not producing the costly enzyme, birch beetles W. (2011). Host plant shifts affect a leaf beetle larvae they had collected from birch can save resources .First and foremost the loss major defense enzyme in Chrysomela trees. “The beetles were able to ingest salicin of salicylaldehyde also means that birch-feeding lapponica. Proceedings of the from willow leaves; salicyl alcohol was also de- populations no longer betray themselves to their National Academy of Sciences USA, tected in their defensive secretions. However, the own enemies anymore, who can trace them be- 108, 4897-4901. alcohol was not transformed to an aldehyde; this cause of the odorous substance. [JWK, AO] PULS/CE 17 4 Newsletter April 2011 | Research Highlight sequence of the respective amino acids, chains Small Genetic Changes with are folded into helices, sheets or other shapes necessary for the polypeptide to perform its bio- Major Consequences logical function. To make sure that enzyme-medi- ated catalysis does not happen in an uncontrolled fashion, many enzymes are regulated by a feed- Plants are continually exposed to herbivore at- back mechanism. In IPMS, this mechanism is lo- tack. To defend themselves, they have developed cated in the last 120 amino acids of the polypep- sophisticated chemical defense mechanisms. tide chain. As soon as enough leucine is available, Plants of the mustard family, such as thale cress the production of leucine precursors is stopped. (Arabidopsis thaliana), produce glucosinolates (mustard oil glucosides) to protect themselves “We found that the missing 120 amino acids not against herbivory. If insect larvae feed on mus- only inactivate the regulation of enzyme activi- tard plants, glucosinolates are hydrolyzed to ties, but also change the architecture of MAM form toxic isothiocyanates. Chemists call this the completely,” Jonathan Gershenzon says. The Plants of the mustard family, such as “mustard oil bomb.” missing 120 amino acids cause a profound change cabbage, produce glucosinolates that in the active site: it expands and becomes able to help to fend off herbivorous insects. Special enzymes are responsible for catalyz- bind larger substrates, allowing it to produce new The enzyme MAM is responsible ing the synthesis of the side chains found in the products: the precursors of glucosinolates. for glucosinolate production. In the various glucosinolates. Jan-Willem de Kraker course of evolution the enzyme IPMS and Jonathan Gershenzon have isolated one of The researchers came across IPMS when they (three dimensional model), which is these enzymes from Arabidopsis thaliana and were looking for genes involved in glucosinolate involved in the formation leucine, was discovered a surprising new insight. The enzyme production. In the context of these studies, they converted to MAM: 120 amino acids methylthioalkylmalate synthase (MAM), which isolated and sequenced the IPMS gene. The scien- disappeared (represented as pale-col- catalyzes glucosinolate formation, strongly re- tists assume that after a duplication of the IPMS ored) and two mutations in the active sembles an enzyme with a completely different DNA sequence millions of years ago, the “twin site of the molecule occurred. Since function: IPMS (isopropylmalate synthase), which DNA” lost the fragment encoding the sequence of then, MAM’s role has been to synthe- is involved in the synthesis of the amino acid leu- the last 120 amino acids. In the course of evolu- size precursors of glucosinolates. cine. The scientists found two major structural tion, this probably happened when the mustard Photo: Angela Schneider, MPI-CE; differences between IPMS and MAM. The last family originated. The loss turned out to be very enzyme model based on Koon, PNAS 120 amino acids in IPMS were absent in MAM, advantageous for the plants: it enabled them to 101, 2004.
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