Influence of confluent holding time on UV light mutagenesis in human diploid fibroblasts

We have investigated the induction of mutants resistant to 6-thioguanine (6TG) following 254 nm ultraviolet light exposure of density-inhibited cultures of human diploid fibroblasts. Phenotypic expression of 6TG resistance was maximal within 9 days and remained stable through 19 days after irradiation. In reconstruction studies, complete recovery of 6TG-resistant mutants occurred at cell densities of up to 35 000 cells per 100-mm petri dish. The induced mutation frequency increased linearly with dose over the range of 3–9 J/m²; the D₀ of the survival curve was 4.2 J/m². Delaying subculture to low density for 1.5–24 h after irradiation produced unexpected alterations in induced mutation frequencies. An increase in UV-induced mutations of approximately 3-fold was observed in cultures maintained in confluence for 3 h. This trend was reversed with longer holding times: the mutation frequency declined sharply in cultures held for 6 h compared to the 3-h value, and thereafter showed a steady and gradual diminution to background levels.

These data suggest that the repair of potentíally mutagenic damage is a complex phenomenon which can lead to an increase or decrease in mutation frequency as a function of holding time. Although the decline in mutation frequency observed following longer holding intervals is consistent with the notion of an error-free process, we hypothesize that the increased mutation frequency produced by a short holding period reflects the existence of a cell-mediated process which enhances the mutagenic potential of at least some UV-induced DNA photoproducts.     

The SOS regulatory system: control of its state by the level of RecA protease

Our current understanding of the SOS regulatory system suggests that it can exist in two extreme states: in the repressed state, LexA protein is active, and it represses a particular set of genes called SOS genes. In the induced state, which results from various impairments to DNA replication, LexA repressor is cleaved by the specific protease activity of the RecA protein; in consequence, the SOS genes are derepressed and they express various functions that are believed to aid cell survival in induced cells. Since high levels of RecA protease activity turn on this system, it seems plausible that the level of protease activity will also control the transitions between the two states of the system. In order to assess the in vivo level of protease activity, antibody techniques were used to study the stability of LexA repressor during various phases of the SOS regulatory cycle. Repressor was reasonably stable in the repressed state, but it was degraded within a few minutes after an inducing treatment. Cleavage depended upon the RecA protease activity and resulted in the same products as seen in vitro. Cleavage preceded, and did not depend upon, derepression of any SOS gene. During the transition to the repressed state, LexA repressor became increasingly stable with time, suggesting that as DNA damage was repaired the level of protease declined. This decline depended upon derepression of the regulatory system, consistent with the belief that an inducing signal, resulting from DNA damage, reversibly activates the RecA protease and is removed by the action of one or more SOS functions. At low levels of DNA damage, a subinduced state was observed in which repressor level was reduced by a low level of cleavage. These data indicate that the level of RecA protease activity controls the state of the system and the transitions between its two states.     

The lateral pectoral composite flap in one-stage reconstruction of the irradiated mandible

We present a composite flap based on the musculature of the anterolateral thorax that allows immediate reconstruction of the mandible through the interposition of vascularized bony fifth rib. While the vascular pedicle remains the pectoral branch of the thoracoacromial artery, the pectoralis major muscle can be spared largely intact. The lateral position of the composite rib segment ensures a purely bony composition, while the lateral skin island remains largely hairless. Despite the unfavorable conditions of massive tumor size and prior irradiation attending the clinical cases presented, the complication rate was acceptable, with only one flap loss, and this followed technical error.     

Structure and function in the excretory systems of some terrestrial prosobranch snails (Cyclophoridae)

The excretory systems of terrestrial prosobranch snails of the family Cyclophoridae, collected in Jamaica, Costa Rica and South Africa, have been examined physiologically and as regards their gross and fine structure. The process of urine formation commences in the heart, where fluid is filtered across the wall of the ventricle. Filtration through the auricular wall is believed to be negligible. The kidney, which contains three types of cell, modifies the composition of the filtrate. One type of resorptive cell, characterized by basal infoldings associated with mitochondria, takes up salts. Another type, with basal subcellular spaces, may be responsible for taking up water. The third type of cell is secretory, producing concretions of uric acid and phospholipid which are liberated into the kidney lumen when the cell degenerates.
The rate and mechanism of urine production have been investigated using injections of inulin. The filtration rate at 25°C is 0.5 μl/g/min, and in 100% R.H. the average rate of urine production is 0–39 μl/g/min.
An accessory excretory organ has been developed from the hypobranchial gland of aquatic forms. It is composed of groups of subepithelial tubular glands opening into the mantle cavity by one or a series of pores, and secreting purines, phospholipids and mucus. There is evidence that this organ becomes progressively more complex in forms occupying drier habitats.
The systems of excretion and osmoregulation in the Cyclophoridae are considered to be very similar to those in their aquatic relatives, the Viviparidae and Ampullariidae. Certainly the cyclophorids are not as well adapted to a terrestrial life as are the Pulmonata, and in many respects they may be considered "aquatic" snails living on land.