Metal ion requirements and other aspects of the reaction catalyzed by M1 RNA, the RNA subunit of ribonuclease P from Escherichia coli.

M1 RNA, the RNA subunit of ribonuclease P from Escherichia coli, can under certain conditions catalytically cleave precursors to tRNA in the absence of C5, the protein moiety of RNase P. M1 RNA itself is not cleaved during the reaction, nor does it form any covalent bonds with its substrate. Only magnesium and, to a lesser extent, manganese ions can function at the catalytic center of M1 RNA. Several other ions either inhibit the binding of magnesium ion at the active site or function as structural counterions. The reaction rate of cleavage of precursors to tRNAs by M1 RNA is enhanced in the presence of poly-(ethylene glycol) or 2-methyl-2,4-pentanediol. Many aspects of the reaction catalyzed by M1 RNA are compatible with a mechanism in which phosphodiester bond cleavage is mediated by metal ion.

Molecular modelling of protein-nucleic acid interactions.

Computer modeling techniques to study the interaction of proteins with nucleic acids are presented. The methods utilize information from genetic and chemical modification experiments and macromolecular structural constraints. These techniques, in addition to computer model building procedures and theoretical energy calculations, are illustrated for the study of the lac and cro repressor-operator systems. Our predicted interactions between lac and its operator agree with those recently reported for lac based upon sequence alignment with the cro repressor. Several molecular models of the putative helical segment of cro interacting with its OR3 operator are presented. These models are reflective of intermediate conformations experienced by the repressor in recognition of the operator sequence. The results of our studies are further discussed in terms of the design of short peptides interacting with nucleic acid sequences and the evolutionary requirements in establishing these repressor interactions.

Organ vascularity and metastatic frequency.

The "hemodynamic" or "mechanical" theory proposes that the frequency of metastases in different organs is primarily determined by the numbers of cancer cells delivered to them in their arterial blood. This theory has not yet been adequately tested in man because reproducible, noninvasive measurements of organ blood flow have only recently become available. Correlation between these data and the metastatic frequency in 10 organs, in groups of patients with primary cancers in 15 anatomic sites, has therefore been sought. No correlation was obtained between metastatic frequency and organ weights, blood volumes, blood volumes per gram, "transit times," or blood flow. However, correlations significant at the 4-8% level were obtained between organ blood flow per gram and metastatic frequency in 4 of 5 groups of primary cancers with initial venous drainage into the portal system, compared with 1 of 10 draining into the caval system. At present, no definitive explanation can be offered for the apparent compliance of one set of primary cancers with the "hemodynamic" theory of metastasis, but not the others.

The goitrogen 3-hydroxy-4(1H)-pyridone, a ruminal metabolite from Leucaena leucocephala: effects in mice and rats.

Mice fed a diet containing 1% (w/w) 3-hydroxy-4(1H)-pyridone (DHP) developed goitre even with a diet high in iodine whereas mimosine (0.5% w/w) did not produce goitre even with a low-iodine diet. Thyroid enlargement was apparent (measured morphometrically) by the 7th week and was advanced by the 11th week. Histologically the goitre was hyperplastic in type. No marked histological changes were found in other organs of mice fed DHP or any organs of mice fed mimosine, except for some atrophy of hair follicles. A single intragastric dose of DHP inhibited the uptake of 125I by the thyroid in the rat but an equivalent dose of mimosine did not. Evidence is presented that the inhibition occurs at the iodine binding step, as with methyl thiouracil, rather than at the iodide trapping step, as with thiocyanate. Chronic treatment of mice with DHP, as with 6-methyl thiouracil, increased the avidity of the thyroid in taking up 125I. The major conjugated form of DHP in mammals, DHP-3-O-glucuronide, was almost as effective a goitrogen as the unconjugated compound when given by mouth but considerably less active than the free form in the blood stream. It was concluded that DHP is a potent antithyroid compound of the thiouracil type with low general toxicity, since mammals can tolerate a level of intake sufficient to produce goitre in spite of iodine supplementation.