A proposed neural pathway for vocalization in South African clawed frogs,Xenopus laevis

Vocalizations of South African clawed frogs are produced by contractions of laryngeal muscles innervated by motor neurons of the caudal medulla (within cranial nerve nucleus IX-X). We have traced afferents to laryngeal motor neurons in male and female frogs using retrograde axonal transport of horseradish peroxidase conjugated to wheat germ agglutinin (HRP-WGA). After iontophoretic injection of HRP-WGA into n. IX-X, retrogradely labelled neurons were seen in the contralateral n. IX-X, in rhombencephalic reticular nuclei, and in the pre-trigeminal nucleus of the dorsal tegmental area (DTAM) of both males and females.     

The structural stability of a protein is an important determinant of its proteolytic susceptibility in Escherichia coli

To investigate the relationship between the degradation rate of a protein in Escherichia coli and its thermal stability in vitro, we constructed a set of variants of the N-terminal domain of lambda repressor with a wide range of melting temperatures. Pulse-chase experiments showed that, within this set, the proteins that are most thermally stable have the longest intracellular half-lives and vice versa. Moreover, second-site mutations which act directly or indirectly to increase the thermodynamic stability of the native N-terminal domain were found to suppress the intracellular degradation of one of the unstable mutants. These data suggest that thermal stability is, indeed, a key determinant of the proteolytic susceptibility of this protein in the cell. It is not the sole determinant, however, as sequences at the extreme C terminus of the N-terminal domain can influence proteolytic sensitivity without affecting the stability of the native structure. We propose that the thermal stability of the N-terminal domain of lambda repressor is an important determinant of its proteolytic sensitivity because degradation proceeds primarily from the unfolded form and that sequence determinants within the unfolded chain influence whether the unfolded protein will be a good substrate for proteolytic enzymes.     

The type IC hsd loci of the enterobacteria are flanked by DNA with high homology to the phage P1 genome: implications for the evolution and spread of DNA restriction systems

Eco R124I, Eco DXXI and Eco prrI are the known members of the type IC family of DNA restriction and modification systems. The first three are carried on large, conjugative plasmids, while Eco prrI is chromosomally encoded. The enzymes are coded by three genes, hsdR , hsdM and hsdS . Analysis of the DNA sequences upstream and downstream of the type IC hsd loci shows that all are highly homologous to each other and also to sequences present in the bacteriophage P1 genome. The upstream sequences include functional phd and doc genes, which encode an addiction system that stabilizes the P1 prophage state, and extend to and beyond pac , the site at which phage DNA packaging begins. Downstream of the hsd loci, P1 DNA sequences begin at exactly the same place for all of the systems. For Eco DXXI and Eco prrI the P1 homology extends for thousands of base pairs while for Eco R124I an IS 1 insertion and an associated deletion have removed most of the P1-homologous sequences. The significance of these results for the evolution of DNA restriction and modification systems is discussed.     

N1-Methyl-2-125I-Lysergic Acid Diethylamide, a Preferred Ligand for In Vitro and In Vivo Characterization of Serotonin Receptors

Methylation of 2-125I-lysergic acid diethylamide (125I-LSD) at the N1 position produces a new derivative, N1-methyl-2-125I-lysergic acid diethylamide (125I-MIL), with improved selectivity and higher affinity for serotonin 5-HT2 receptors. In rat frontal cortex homogenates, specific binding of 125I-MIL represents 80-90% of total binding, and the apparent dissociation constant (KD) for serotonin 5-HT2 receptors is 0.14 nM (using 2 mg of tissue/ml). 125I-MIL also displays a high affinity for serotonin 5-HT1C receptors, with an apparent dissociation constant of 0.41 nM at this site. 125I-MIL exhibits at least 60-fold higher affinity for serotonin 5-HT2 receptors than for other classes of neurotransmitter receptors, with the dopamine D2 receptor as its most potent secondary binding site. Studies of the association and dissociation kinetics of 125I-MIL reveal a strong temperature dependence, with very slow association and dissociation rates at 0 degree C. Autoradiographic experiments confirm the improved specificity of 125I-MIL. Selective labeling of serotonin receptors was observed in all brain areas examined. In vivo binding studies in mice indicate that 125I-MIL is the best serotonin receptor label yet described, with the highest frontal cortex to cerebellum ratio of any serotonergic radioligand. 125I-MIL is a promising ligand for both in vitro and in vivo labeling of serotonin receptors in the mammalian brain.     

Evidence for a homolog of the yeast cell cycle regulatory gene product of cdc2+ in Physarum polycephalum

Evidence for the presence of a Cdc2-like protein in Physarum polycephalum has been obtained using a peptide antibody directed against a highly conserved amino acid sequence near the N-terminal end of Cdc2, Cdc28 and Cdc2HS. The antibody detected a 34 kDa cytoplasmic protein, similar in apparent size to Cdc2 in yeast and Cdc2Hs in HeLa cells. A 60 kDa nuclear band was also detected in Physarum but not in yeast or HeLa. Evidence is presented that this is not related to the 34 kDa protein nor is it found in HeLa nuclei or yeast cells. The Cdc2-like protein level did not fluctuate over more than 10 h of the naturally synchronous cell cycle of Physarum. Several heat-shock experiments using regimens that either: delayed mitosis and S-phase; prevented mitosis or uncoupled S-phase from mitosis were performed. None had any effect on the level of the Cdc2-like protein. The induction of spherulation by starvation was shown to have no effect on the levels of the 34 kDa Cdc2 analog. The invariant level of the 34 kDa protein during the cell cycle and starvation is consistent with previous results obtained with yeast. Three heat-shock regimens which either delay mitosis, eliminate S-phase or uncouple mitosis from S-phase in Physarum also had no effect on the level of the 34 kDa protein. This result emphasizes the stable nature of this protein.