The herpes simplex virus type 1 (HSV-1) a sequence is present as a direct repeat at the two termini of the 152-kilobase viral genome and as an inverted repeat at the junction of the two unique components L and S. During replication, the HSV-1 genome undergoes inversion of L and S, producing an equimolar mixture of the four possible isomers. Isomerization is believed to result from recombination triggered by breakage at the a sequence, a recombinational hot spot. We have identified an enzyme in HeLa cell extracts that preferentially cleaves the a sequence and have purified it to near homogeneity. Microsequencing showed it to be human endonuclease G, an enzyme with a strong preference for G؉C-rich sequences. Endonuclease G appears to be the only cellular enzyme that can specifically cleave the a sequence. Endonuclease G also showed the predicted recombination properties in an in vitro recombination assay. Based on these findings, we propose that endonuclease G initiates the a sequence-mediated inversion of the L and S components during HSV-1 DNA replication.The genome of herpes simplex virus type 1 (HSV-1) 1 consists of a linear 152-kb double stranded DNA molecule composed of two unique segments, U L (unique long) and U S (unique short), with each segment flanked by inverted repeated sequences. U L is flanked by the ab and bЈaЈ sequences, whereas U S is flanked by aЈcЈ and ca sequences, with the aЈ sequence shared by U L and U S (aЈ, bЈ and cЈ are inverted repeats of a, b, and c). The organization of the HSV-1 genome can therefore be delineated as ab-U L -bЈaЈcЈ-U S -ca (1). HSV-1 is known to freely invert the U L and U S segments relative to each other to generate four isomers (2-4), an event that is closely associated with viral DNA replication (5, 6). The four isomers exist in an equimolar ratio, indicative of 100% recombination frequency. Thus, underlying inversion is a very efficient recombination reaction. It has been proposed that the inversion event results from double strand break repair initiated by multiple double strand breaks in the inverted repeats of the HSV-1 genome (4). Although the manner in which double strand breaks are generated is unknown, the a sequence appears to be involved. The a sequence has been shown to be sufficient for the U L -U S inversion (7); however, it is not clear whether or not it is dispensable (8).The a sequence is ϳ300 bp long, containing 83% GϩC, and is itself composed of multiple repeated sequences (see Fig. 1) consisting of 20-bp direct repeats (DR1) at each end followed by two unique regions (U b and U c ) separated by multiple copies of the 12-bp DR2 repeats (7, 9 -11). In addition to its role in recombination, the a sequence also contains sites for cleavage and packaging of the concatameric product of rolling circle DNA replication (7,12).Recombination mediated by the a sequence could be reproduced in a plasmid-based recombination system, which demonstrated that the a sequence is a recombinational hot spot (13,14). More importantly, studies with this system showed th...

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This video submission presents the experimental validation and testing of a leg-wheel hybrid mobile robot Quattroped. By combining the smooth and efficient motion of wheels on the flat ground with the great mobility of legs on rough terrains, the design of the robot aims for agile and versatile yet efficient locomotion in both natural and artificial environments. Compared to most hybrid platforms, which have separate mechanisms of wheels and legs, this robot is implemented with a transformation mechanism that directly changes the morphology of wheels (i.e., a full circle) into half-circle legs, each with 2 active degrees-of-freedom (i.e., combining two half circles as a leg ). The experimental testing includes flat terrain driving and turning in the wheeled mode, leg-wheel mode switching, and step crossing, bar crossing, natural rough terrain walking, and stair climbing in the legged mode.

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