The Aspergillus nidulans phytochrome FphA represses sexual development in red light

SummaryPhytochrome photoreceptors sense red and far-red light through photointerconversion between two stable conformations, a process mediated by a linear tetrapyrrole chromophore [1]. Originally, phytochromes were thought to be confined to photosynthetic organisms including cyanobacteria, but they have been recently discovered in heterotrophic bacteria and fungi, where little is known about their functions [2, 3]. It was shown previously in the ascomycetous fungus Aspergillus nidulans that asexual sporulation is stimulated and sexual development repressed by red light [4]. The effect was reminiscent of a phytochrome response, and indeed phytochrome-like proteins were detected in several fungal genomes [5]. All fungal homologs are more similar to bacterial than plant phytochromes and have multifunctional domains where the phytochrome region and histidine kinase domain are combined in a single protein with a C-terminal response-regulator domain. Here, we show that the A. nidulans phytochrome FphA binds a biliverdin chromophore, acts as a red-light sensor, and represses sexual development under red-light conditions. FphA-GFP is cytoplasmic and excluded from the nuclei, suggesting that red-light photoperception occurs in the cytoplasm. This is the first phytochrome experimentally characterized outside the plant and bacterial kingdoms and the second type of fungal protein identified that functions in photoperception

Antagonism of receptors for gastrin, cholecystokinin and GRP/bombesin in postprandial stimulation of exocrine pancreas in dogs

Postprandial pancreatic secretion results from the interaction of neural and hormonal factors such as cholecystokinin (CCK), gastrin and gastrin releasing peptide (GRP), but their contribution to the net secretion is not established. Recent description of highly specific and potent hormonal receptor antagonists allows the determination of the physiological role of CCK, gastrin and GRP. In six dogs with chronic pancreatic fistulas, the blockade of CCK receptors by L-364, 718, gastrin receptors by L-365, 260 or GRP/bombesin receptors by nonapeptide RC-3095 failed to affect basal or sham-feeding induced pancreatic secretion indicating that none of these hormonal peptides plays a major role in this secretion. In contrast, the pancreatic response to ordinary feeding (which includes cephalic, gastric and intestinal phases), that was accompanied by a significant increment in plasma CCK and gastrin levels, was strongly inhibited (by over 50%) by L-364, 718 and slightly (by 20-30%) by L-365, 260 but not by RC-3095. Each antagonist was given at a dose that eliminated the secretory response to CCK, gastrin or GRP, respectively. We conclude that specific receptor antagonists are useful tools in assessing the physiological role of gut hormones in the control of pancreatic secretion and that none of the peptides tested appears to be involved in the cephalic phase. However, CCK plays a major role in the postprandial stimulation of pancreatic secretion

The hybrid histidine kinase Slr1759 of the cyanobacterium Synechocystis sp. PCC 6803 contains FAD at its PAS domain

Michel K-P, Schröder A-K, Zimmermann M, et al. The hybrid histidine kinase Slr1759 of the cyanobacterium Synechocystis sp. PCC 6803 contains FAD at its PAS domain. Archives of Microbiology. 2009;191(6):553-559.The cyanobacterium Synechocystis sp. PCC 6803 harbours 47 histidine kinases (Hiks). Among these are hybrid histidine kinases with one or two response regulator domains as well as numerous Hiks with several sensory domains. One example is the hybrid histidine kinase Slr1759 (Hik14) that has two PAS domains arranged in tandem linked to a predicted GAF domain. Here, we show that a Slr1759 derivative recombinantly expressed in Escherichia coli has a flavin cofactor. Using truncated Slr1759 variants, it is shown that the flavin associates with the first PAS domain. The cofactor reconstitutes the activity of d -amino acid oxidase apoprotein from pig kidney, indicating that the flavin derivative is FAD. Furthermore, the Slr1759 histidine kinase domain indeed undergoes autophosphorylation in vitro. The phosphorylated product of a recombinant Slr1759 derivative is sensitive to acids, pointing to a histidine residue as the phosphate-accepting group