This Week in PNAS
BIOPHYSICS
Prokaryotic proton pumping in eukaryotes
Stephen Waschuk et al. report that the fungus Leptosphaeria maculans possesses a rhodopsin protein able to translocate protons across membranes in response to light, a function thought to be unique to prokaryotes. Traditionally, respiration and chlorophyll-based photosynthesis are the two methods used by eukaryotes to create a proton gradient, whereas archaebacteria undergo retinal-based photosynthesis dependent on the membrane protein bacteriorhodopsin. Although rhodopsin proteins have been found in fungi and algae, they appear to function only in a photosensing manner. According to the authors, however, L. maculans appears to possess a rhodopsin with high sequence homology to bacteriorhodopsin's membrane-spanning regions. Time-resolved laser spectroscopy of the fungal opsin expressed in Pichia revealed rapid kinetics relative to the slow photocycle of previously characterized fungal opsins. To test the ability of Leptosphaeria rhodopsin to function as a protein pump, the researchers embedded the protein in liposomes. In the presence of light, the authors noted a change of pH between the inside and outside of the liposome. Some lower eukaryotes may thus share the prokaryotic ability to use light-sensitive retinal proteins as electrical gradient generators.
Light-induced absorption changes of Leptosphaeria rhodopsin.
“Leptosphaeria rhodopsin: Bacteriorhodopsin-like proton pump from a eukaryote” by Stephen A. Waschuk, Arandi G. Bezerra, Jr., Lichi Shi, and Leonid S. Brown (see pages 6879–6883)
PHYSIOLOGY
Estrogenic chemicals linked to mouse prostate deformities
Barry Timms et al. report that male mouse fetuses exposed to the estrogenic chemicals ethinylestradiol and bisphenol A developed deformities in the prostate and urethra. Ethinylestradiol is used in oral contraceptives, and bisphenol A is present in polycarbonate plastic products and the lining of tin cans. Timms et al. fed ethinylestradiol and bisphenol A to pregnant CD-1 mice and found that both compounds produced an increase in number and size of dorsolateral prostate ducts in male fetuses, as well as an overall increase in prostate duct volume. These increases appeared to be caused by heightened proliferation of basal epithelial cells in the primary ducts. Exposure to the estrogenic chemicals also caused malformation of the urethra in the colliculus region, with a narrowing of the urethra at the bladder neck. The authors showed that these adverse effects were identical to those caused by a similar dose of the estrogenic drug diethylstilbestrol, a known carcinogen. According to the authors, women who become pregnant despite using oral contraceptives may pass ethinylestradiol to their fetus. Also, the authors note that bisphenol A may leach into food and beverages from tin cans and polycarbonate plastic containers.
Bisphenol A effect on fetal development.
“Estrogenic chemicals in plastic and oral contraceptives disrupt development of the fetal mouse prostate and urethra” by Barry G. Timms, Kembra L. Howdeshell, Lesley Barton, Sarahann Bradley, Catherine A. Richter, and Frederick S. vom Saal (see pages 7014–7019)
PLANT BIOLOGY
Cold disrupts chestnut circadian clock
According to Alberto Ramos et al., the circadian rhythm of the chestnut tree enters dormancy during winter. Dormancy is an important adaptive strategy for plants to survive stressful environmental conditions. Ramos et al. investigated the role of the chestnut's circadian clock genes in regulating cold-induced dormancy. The authors collected European chestnut samples during December, as well as chilled chestnut seedlings, and analyzed the expression of two marker genes, CsTOC1 and CsLHY. These genes are homologous to components of the circadian oscillator in Arabidopsis. The authors found that, like their counterparts in Arabidopsis, mRNA expression levels of CsTOC1 and CsLHY fluctuated in a clock fashion during simulated summer days. During winter days, however, mRNA expression ceased to oscillate and remained high, indicating an alteration in the circadian clock. The same effect was observed when the researchers chilled the seedlings to 4°C; oscillation resumed as temperatures were raised. Ramos et al. suggest that the different behavior of chestnut and Arabidopsis circadian clocks under cold temperatures indicates that cold acclimation in temperate woody plants may have significant specific features, compared with annual herbaceous plants.
“Winter disruption of the circadian clock in chestnut” by Alberto Ramos, Estefanía Pérez-Solís, Cristian Ibáñez, Rosa Casado, Carmen Collada, Luis Gómez, Cipriano Aragoncillo, and Isabel Allona (see pages 7037–7042)
SOCIAL SCIENCES
Altruistic punishment may explain origin of cooperation
Why humans frequently cooperate with genetically unrelated strangers is puzzling, because natural selection discourages such personally costly behavior. However, cooperation may make sense in society with altruistic punishment, in which people pay a cost to punish free-riders in public goods games. Natural selection would again seem to favor those who free-ride on the cooperative benefits generated by altruistic punishers, so how this behavior could evolve is not clear. To examine how altruistic punishment could take root in a society, James Fowler developed an evolutionary model of a population of altruistic punishers, contributors, defectors, and nonparticipants. He found altruistic punishers can enter a population and change the dynamics of the group. Under certain conditions, altruistic punishment is so beneficial to the population that it will come to dominate the behavior of the group and keep noncooperators at bay. The model suggests that the usual cycle of cooperation, defection, and nonparticipation identified by other researchers does not hold when a punisher enters the population. Fowler's model also suggests there are restrictions on the kinds of strategies that punishment can enforce.
Population dynamics with altruistic punishers.
“Altruistic punishment and the origin of cooperation” by James H. Fowler (see pages 7047–7049)
