Assessment of a model for intron RNA secondary structure relevant to RNA self-splicing--a review

A widespread class of introns is characterized by a particular RNA secondary structure, based upon four conserved nucleotide sequences. Among such "class I" introns are found the majority of introns in fungal mitochondrial genes and the self-splicing intron of the large ribosomal RNA of several species of Tetrahymena. A model of the RNA secondary structure, which must underlie the self-splicing activity, is here evaluated in the light of data on 16 further introns. The main body or "core structure" of the intron always consists of the base-paired regions P3 to P9 with the associated single-stranded loops, with P2 present also in most cases. Two minority sub-classes of core structure occur, one of which is typical of introns in fungal ribosomal RNA. Introns in which the core structure is close to the 5' splice site all have an internal guide sequence (IGS) which can pair with exon sequences adjacent to the 5' and 3' splice sites to align them precisely, as proposed by Davies et al. [Nature 300 (1982) 719-724]. In these cases, the internal guide model allows us to predict correctly the exact location of splice sites. All other introns probably use other mechanisms of alignment. This analysis provides strong support for the RNA splicing model which we have developed.     

Home births in England and Wales, 1979: perinatal mortality according to intended place of delivery.

A survey was carried out of all 8856 births occurring at home in England and Wales in 1979. Of these births, 67% had been booked for delivery at home, 21% had been booked for delivery in hospital, 3% had not been booked, and for 9% the intended place of delivery was unknown. The perinatal mortality varied almost 50-fold according to the intended place of delivery, ranging from 4.1/1000 births in those booked for delivery at home to 196.6/1000 unbooked births. Deliveries that occurred at home but had been booked for a hospital consultant unit were associated with a perinatal mortality of 67.5/1000. Births that had been booked for delivery at home included the smallest proportion of babies of low birth weight: 2.5% weighed 2500 g or less compared with 18% of those booked for consultant units and 29% of those not booked. Within these low birthweight groups there were noticeable differences in perinatal mortality; births booked to occur at home had the lowest mortality and unbooked births had the highest. Perinatal mortality among babies who weighed more than 2500 g was generally low irrespective of the intended place of delivery; the only exception was in babies whose delivery had not been booked. In all groups perinatal mortality was considerably higher in nulliparous than parous women. Women booking a delivery at home are clearly a selected group, and some may have been transferred to hospital during labour and were thus not included in the survey. Nevertheless, these data suggest that the perinatal mortality among births booked to occur at home is low, especially for parous women.     

Preparation of internally labelled rat pituitary somatotropin (growth hormone).

Rat somatotropin (growth hormone) was labelled biosynthetically by incubating anterior pituitary lobes with radioactive amino acids for 24 h in a simple buffered salts medium containing glucose. The labelled hormone was isolated by preparative polyacrylamide-gel electrophoresis or by chromatography on Sephadex G-100 and then DEAE-cellulose. The labelled material was pure by several criteria and cross-reacted immunologically with unlabelled rat somatotropin. When a mixture of 14C-labelled amino acids was used for labelling the protein, label could be introduced into these same amino acids of somatotropin, though relative specific radioactivities varied considerably. Somatotropin labelled by the procedures described in the present paper was suitable for structural studies and could be used for a variety of other biochemical experiments.     

The peritrophic membrane in adult simuliids (Diptera) before and after feeding on blood and blood-sucrose mixtures

A peritrophic membrane formed in the posterior midgut of female simuliids of seven species within 30 min of feeding on blood of various avian or mammalian hosts, and 6–12 hr thereafter it displayed up to seven concentric laminae. Within the first 48 hr of blood digestion, the posterior part of the peritrophic membrane (where digestion was most advanced) began to decompose and PM disappearance was complete at the end of blood digestion. In different species blood digestion at 18°–20° required 120–180 hr which was increased to over 200 hr by microsporidan infection. If a blood-sucrose mixture went directly to the midgut, a thin membrane formed around it, but not if it went indirectly by way of the crop. A prefeeding secretion in the anterior midgut is considered to be the peritrophic membrane of the pharate adult surrounding a small, but variable, amount of material, the meconium.

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Résumé Sept espèces de simulies ont été étudiées: 5 ornithophiles, 2 mammalophiles. Trente minutes après un repas de sang sur divers oiseaux (poulet, canard) ou mammifères (homme, cerf, élan d'Amérique) on constate qu'une membrane péritrophique est sécrétée par toute le surface épithéliale dans la région moyenne de l'intestin moyen postérieur dilaté des femelles. Six à douze heures après le repas cette membrane comporte jusqu'à sept lamelles concentriques. Une sécrétion moins importante se forme autour de la petite quantité de sang que l'on trouve dans l'estomac antérieur, non dilaté et étroit; cette sécrétion glisse vers l'arrière et forme une capsule recouvrant le pôle antérieur du bol alimentaire sanguin dans la partie postérieure de l'intestin moyen, ceci pendant le premier jour. La digestion commence à la périphérie et à l'extrémité postérieure de la masse sanguine et se poursuit enfin à l'extrémité antérieure, où la membrane péritrophique conserve le plus longtemps sa structure en contact avec le sang non digéré. Dans les premières 48 heures de la digestion du sang, la région postértieure de la membrane péritrophique (là où la digestion est la plus avancée) commence à se décomposer, et sa disparition est complète quand la digestion du sang est achevée. Chez les différentes espèces étudiées, la digestion du sang à 18–20° exige 120–180 heures, pouvant demander plus de 200 heures chez des insectes atteints d'une infection par des microsporidies. Si un mélange de sang et sucrose parvient directement dans la région moyenne de l'intestin moyen, une membrane mince se forme autour, mais non si cet aliment passe par le jabot. Une sécrétion précédant l'alimentation s'observe dans l'intestin moyen antérieur des mouches non nourries et doit représenter, chez l'adulte venant d'éclore, la membrane péritrophique entourant le méconium: petite quantité de matière d'importance variable. Cette matière se retrouve souvent au centre de la masse sanguine après alimentation et est lente à digérer.

     

In vitro secondary MLR. I. Kinetics of proliferation and specificity of in vitro primed responder cells.

We have examined the kinetics and specificity of secondary in vitro mixed lymphocyte reactions (MLR). With limited numbers of primed responder cells (PRC) in the presence of "excess antigen" it was possible to obtain proliferative responses that were proportional to the number of PRC initially placed in culture. The responding cells, after an initial lag period, seem to grow exponentially until day 3 of culture. The responses of PRC (with the strain combinations and culture conditions described in this report) seemed to be directed toward stimulator cell determinants whose expression was determined by genes in the I region of the MHC. In one case, the relevant incompatibilities could be further restricted to the I-A region. Although PRC responded best to stimulator cells sharing the I region with the priming stimulator cell, apparent cross-reactivity could be observed by restimulating PRC with stimulator cells that did not carry the MHC haplotype of the priming stimulator cell. The rate of proliferation (measured as 3H-thymidine incorporation) in these apparent cross-reactions was reproducible and comparable to the rate observed in response to the priming stimulator cell. It was possible, therefore, to estimate the proportion of PRC that reacted in the presence of third party stimulator cells compared to the response of these PRC to the priming stimulator cells. We have estimated that the response of A (B6) PRC against H-2d and H-2s haplotype stimulator cells is about half of the response of these PRC to H-2b, the priming stimulator cell.     

A simplified method for mapping deletion/substitution mutants of bacteriophage lambda.

A convenient and rapid method for mapping deletion/substitution mutants of phage λ is presented. The method involves: (a) forming heteroduplex DNA molecules between the wild-type and the mutant: (b) digestion of the single-stranded regions with S₁ nuclease; (c) cleavage of a portion of the remaining duplex DNA with EcoRI nuclease or any convenient restriction endonuclease; and (d) separation of the resulting DNA fragments by gel electrophoresis. Using three deletion/substitution mutants with known endpoints, we show that the values obtained by this method deviate, on the average, by ±120 base pairs from the values obtained by electron microscopy.