High-resolution proton magnetic resonance studies of the 3'-terminal colicin fragment of 16 S ribosomal RNA from Escherichia coli. Assignment of iminoproton resonances by nuclear Overhauser effect experiments and the influence of adenine dimethylation on the hairpin conformation

The "colicin" fragments comprising the 49 3'-terminal nucleotides of 16 S ribosomal RNA have been isolated from wild-type Escherichia coli and from a kasugamycin-resistant mutant that lacks methylation of two geminal adenine residues. Proton nuclear magnetic resonance (n.m.r.) spectra (500 MHz) were recorded at various temperatures. The low-field resonances arising from the hydrogen-bonded iminoprotons of paired bases were assigned using the nuclear Overhauser effect (n.o.e.). Crucial to the interpretation of the spectra are the resonances that originate from the two hydrogen-bonded iminoprotons of a U X G basepair. Combined with temperature-jump relaxation kinetics experiments the n.o.e.s lead to the conclusion that a conserved A X U/U X G junction in the hairpin is a thermolabile dislocation in the helix. The n.m.r. spectra of the wild-type and mutant fragment are only different with respect to the iminoproton resonances of the two base-pairs adjoining the hairpin loop. The spectra recorded at various temperatures tend to indicate that dimethylation of the adenosines labilizes these base-pairs, but no definitive conclusions are drawn. The results confirm our previous views that dimethylation of the adenosine residues affects the conformation of the hairpin loop.     

Microtubule-associated protein MAP2 preferentially binds to a dA/dT sequence present in mouse satellite DNA

Microtubule-associated protein MAP2 binds to the Sau96.1 restriction monomer fragment of mouse satellite DNA. This fragment is also present in a lower proportion in bulk DNA. The digestion of MAP2-Sau96.1 fragment complex by DNase results in the protection of certain nucleotide sequences. The sequence poly(dA)4/poly(dT)4 is mainly protected against DNase digestion.     

“Ischaemic” Colitis in Young Adults

Four cases of transient haemorrhagic colitis seen in young adults resemble 10 others in the literature. The 14 cases are distinguished from the classical transient ischaemic colitis syndrome by their youth, the low incidence of stricture formation, and the prevalence of right-sided lesions.     

An evaluation of four methods for measuring cholesterol absorption by the intestine in man

Critical comparisons have been made in 12 patients of four methods for measuring cholesterol absorption from the intestine. Methods I-III depend on the use of labeled cholesterol (intravenously or continuous labeling orally) in conjunction with sterol balance measurements; Method IV can be carried out with only a single test dose containing labeled cholesterol plus labeled beta-sitosterol. In the latter technique absorption is calculated as the loss of cholesterol relative to beta-sitosterol during intestinal transit. Method III (isotopic steady-state method) proved to be undependable because of uncertainties in determining the existence of an isotopic steady state. However, Method IV gave good agreement with Methods I and II, and it appears to have certain practical as well as theoretical advantages. Although Method IV requires collections of stools for up to 8 days, it is nevertheless the most rapid and the simplest of all the methods for estimating absorption. It can also be used in certain situations, such as in fur-licking animals, when Methods I and II are inadequate. Therefore, this method would seem to be a valuable addition to other isotopic techniques for estimating cholesterol absorption in man.