Physiological importance of a circadian clock outside the suprachiasmatic nucleus.

Circadian clocks are widely distributed in mammalian tissues, but little is known about the physiological functions of clocks outside the suprachiasmatic nucleus of the brain. The retina has an intrinsic circadian clock, but its importance for vision is unknown. Here, we show that mice lacking Bmal1, a gene required for clock function, had abnormal retinal transcriptional responses to light and defective inner retinal electrical responses to light, but normal photoreceptor responses to light and retinas that appeared structurally normal as observed by light and electron microscopy. We generated mice with a retina-specific genetic deletion of Bmal1, and they had defects of retinal visual physiology essentially identical to those of mice lacking Bmal1 in all tissues and lacked a circadian rhythm of inner retinal electrical responses to light. Our findings indicate that the intrinsic circadian clock of the retina regulates retinal visual processing in vivo.

Car: a cytoplasmic sensor responsible for arginine chemotaxis in the archaeon Halobacterium salinarum.

A new metabolic signaling pathway for arginine, both a chemoeffector and a fermentative energy source, is described for Halobacterium salinarum. Systematic screening of 80+ potentially chemotactic compounds with two behavioral assays identified leucine, isoleucine, valine, methionine, cysteine, arginine and several peptides as strong chemoattractants. Deletion analysis of a number of potential halobacterial transducer genes led to the identification of Car, a specific cytoplasmic arginine transducer which lacks transmembrane helices and was biochemically shown to be localized in the cytoplasm. Flow assays were used to show specific adaptive responses to arginine and ornithine in wild-type but not Deltacar cells, demonstrating the role of Car in sensing arginine. The signaling pathway from external arginine to the flagellar motor of the cell involves an arginine:ornithine antiporter which was quantitatively characterized for its transport kinetics and inhibitors. By compiling the chemotactic behavior, the adaptive responses and the characteristics of the arginine:ornithine antiporter to arginine and its analogs, we now understand how the combination of arginine uptake and its metabolic conversion is required to build an effective sensing system. In both bacteria and the archaea this is the first chemoeffector molecule of a soluble methylatable transducer to be identified.

A family of halobacterial transducer proteins.

A DNA probe to the signaling domain of a halobacterial transducer for phototaxis (HtrI) was used to clone and sequence four members of a new family of transducer proteins (Htps) in Halobacterium salinarium potentially involved in chemo- or phototactic signal transduction. The signaling domains in these proteins have 31-43% identity when compared with each other or with their bacterial analogs, the methyl-accepting chemotaxis proteins. An additional region of homology found in three of the Htps has 31-43% identity with HtrI. The Htps contain from 0 to 3 transmembrane helices and Western blotting showed that HtpIII is soluble. The arrangement of the domains in these Htps suggests a modular architecture in their construction.

Stereoselectivity of pigment exchange with 13(2)-hydroxylated tetrapyrroles in reaction centers of Rhodobacter sphaeroides R26.

Bacteriochlorophyll a and bacteriopheophytin a carry a stereochemically labile asymmetric carbon at position C13(2). The steric requirements of photosynthetic reaction centers from Rhodobacter sphaeroides R26 have been probed by exchange experiments with the respective epimeric 13(2)-hydroxylated pigments, in which epimerisation is blocked. (13(2)S)-13(2)-Hydroxy-bacteriochlorophyll a is accepted at both monomeric binding sites, BA,B, (13(2)S)-13(2)-hydroxy-bacteriopheophytin a exclusively at the inactive site HB. The orientation of the 13(2)-COOCH3 substituents in these pigments is the same as in the native (13(2)R)-bacteriochlorophylls and (13(2)R)-bacteriopheophytins. In no cases are the unnaturally configured 13(2)-hydroxylated (13(2)R)-epimers accepted, even if a large excess (> 95%) is offered. It is concluded that the three binding sites always require the 13(2)-COOCH3 group on the opposite side of the macrocycle (down) than the 17-propionic ester side chain (up).