Codon reassignment and the evolving genetic code: problems and pitfalls in post-genome analysis.

The in silico translation of open reading frames, using the 'universal genetic code', must be approached with caution. The uncovering of a number of codon reassignments in nuclear and organellar genomes highlights the importance of experimentally confirming the assignments of all 64 codons for the species whose genome is under investigation. Such alterations to codon meaning also suggest that the genetic code is not 'frozen' and continues to evolve.

Purification and properties of an enterotoxin from a coatless spore mutant of Clostridium perfringens type A.

A method is described for isolating an enterotoxin from a coatless spore mutant (8-6) of Clostridium perfringens type A. The characteristics of this enterotoxin only slightly resembled those of previously isolated enterotoxins of C. perfringens. The type A (8-6) enterotoxin was found to be composed of two subunits of Mr 18 000 with isoelectric points of 3.8 and 4.3. The LD50 for mice was 39 micrograms/kg with 0.10 micrograms corresponding to one erythemal unit. The type A (8-6) enterotoxin was inactivated by heating for 10 min at 60 degrees C. The amino acid composition data of type A (8-6) and delta toxins was similar, but type A (8-6) and type A enterotoxins showed less similarity. This lack of similarity between type A and type A (8-6) enterotoxins was confirmed by the failure of anti-sera to type A enterotoxin to neutralize the type A (8-6) enterotoxin, in both the mouse and erythemal tests.

Low-frequency motion in membranes. The effect of cholesterol and proteins.

Nuclear magnetic resonance (NMR) relaxation techniques have been used to study the effect of lipid-protein interactions on the dynamics of membrane lipids. Proton enhanced (PE) 13C-NMR measurements are reported for the methylene chain resonances in red blood cell membranes and their lipid extracts. For comparison similar measurements have been made of phospholipid dispersions containing cholesterol and the polypeptide gramicidin A+. It is found that the spin-lattice relaxation time in the rotating reference frame (T1 rho) is far more sensitive to protein, gramicidin A+ or cholesterol content than is the laboratory frame relaxation time (T1). Based on this data it is concluded that the addition of the second component to a lipid bilayer produces a low-frequency motion in the region of 10(5) to 10(7) Hz within the membrane lipid. The T1 rho for the superimposed resonance peaks derived from all parts of the phospholipid chain are all influenced in the same manner suggesting that the low frequency motion involves collective movements of large segments of the hydrocarbon chain. Because of the molecular co-operativity implied in this type of motion and the greater sensitivity of T1 rho to the effects of lipid-protein interactions generally, it is proposed that these low-frequency perturbations are felt at a greater distance from the protein than those at higher frequencies which dominate T1.

Effects of dietary supplements of protected lipids on the concentration and transport of beta-carotene and cholesterol in bovine blood and milk: unusual chromatographic behaviour of the high-density lipoprotein with high levels of beta-carotene.

The effects of feeding lipids protected against microbial degradation in the rumen, on the metabolism of beta-carotene and cholesterol in the blood and milk of cows were studied. The diets fed to the cows consisted of a basal mixture of crushed oats and lucerne hay with a protected vitamin supplement containing a-tocopheryl acetate and beta-carotene fed in conjunction with either (i) protected sunflower oil-seed rich in linoleic acid (PO), (ii) protected tallow (PT), or (iii) formaldehyde-treated casein (C) as a control. Diets PO and PT raised the concentrations of beta-carotene and cholesterol in the blood plasma over that observed for diet C. Milk cholesterol concentrations were not affected by dietary supplements, but the level of beta-carotene in milk of cows on diet PO showed a tendency fo fall compared with milk from cows fed PT or C. The properties of the high density lipoprotein (HDLP) of the blood plasma which contained the beta-carotene were affected by the PO diet. As a result of feeding this diet, the fatty-acid composition of the HDLP was altered and it emerged from a gel-filtration chromatographic column earlier than the control. This change in chromatographic behaviour was used as a measure of the effect of the diet, which for some cows, was apparent long after the diet was changed. It is suggested that the altered lipid composition resulting from the PO diet affected the distribution of particle sizes of the HDLP and might interfere with the transfer of beta-carotene from plasma to milk.

Interaction of water with egg lecithin in benzene solution.

Benzene solutions of purified egg lecithin, with small amounts of water added, have been examined by 60 MGz and 100 MHz NMR spectroscopy, infrared spectrophotometry and phase contrast microscopy. The transverse relaxation times of the water, N-methyl and O--H protons are dependent on water concentration. This dependence changes sharply for the water proton at a level of one water molecule per lecithin monohydrate molecule. These results do not fully agree with those reported by other workers. Four mathematical models are examined which could account for the behaviour of the water protons. Models which assume a constant transverse relaxation time for water protons above a level of one water molecule per lecithin molecule cannot predict the behaviour observed. It is sufficient to assume that water protons above this concentration have a single relaxation time which is a linear function of water concentration. The added water associates primarily with the phosphate in the lecithin head group. Above nine water molecules per lecithin monohydrate molecule free water is present in the system.