Rolling-circle transposons in eukaryotes

All eukaryotic DNA transposons reported so far belong to a single category of elements transposed by the so-called “cut-and-paste” mechanism. Here, we report a previously unknown category of eukaryotic DNA transposons, Helitron, which transpose by rolling-circle replication. Autonomous Helitrons encode a 5′-to-3′ DNA helicase and nuclease/ligase similar to those encoded by known rolling-circle replicons. Helitron-like transposons have conservative 5′-TC and CTRR-3′ termini and do not have terminal inverted repeats. They contain 16- to 20-bp hairpins separated by 10–12 nucleotides from the 3′-end and transpose precisely between the 5′-A and T-3′, with no modifications of the AT target sites. Together with their multiple diverged nonautonomous descendants, Helitrons constitute ≈2% of both the Arabidopsis thaliana and Caenorhabditis elegans genomes and also colonize the Oriza sativa genome. Sequence conservation suggests that Helitrons continue to be transposed.

Rolling replication of short DNA circles.

Natural genes and proteins often contain tandemly repeated sequence motifs that dramatically increase physiological specificity and activity. Given the selective value of such repeats, it is likely that several different mechanisms have been responsible for their generation. One mechanism that has been shown to generate relatively long tandem repeats (in the kilobase range) is rolling circle replication. In this communication, we demonstrate that rolling circle synthesis in a simple enzymatic system can produce tandem repeats of monomers as short as 34 bp. In addition to suggesting possible origins for natural tandem repeats, these observations provide a facile means for constructing libraries of repeated motifs for use in "in vitro evolution" experiments designed to select molecules with defined biological or chemical properties.

Rolling resistance of tires measured under transient and equilibrium conditions on Calspan's tire research. Final report.

Transportation Department, Office of the Assistant Secretary for Systems Development and Technology, Washington, D.C.

Rolling resistance of light truck tires. Final report.

National Highway Traffic Safety Administration, Office of Research and Development, Washington, D.C.

Equilibrium and transient rolling resistance of truck tires measured on Calspan's tire research facility. Final report.

Transportation Department, Office of Noise Abatement, Washington, D.C.

Results of energy-loss measurements on passenger car tires operating in the free-rolling and braking/traction modes. Final report.

Transportation Systems Center, Cambridge, Mass.

Laboratory and full-scale parametric studies of the rolling resistance of radial- and bias-ply heavy service truck tires. Final report.

National Highway Traffic Safety Administration, Office of Heavy Duty Vehicle Research, Washington, D.C.

Results of rolling resistance measurements on twelve radial passenger car tires.

National Highway Traffic Safety Administration, Office of Research and Development, Washington, D.C.

Conference on tire rolling resistance, Akron, Ohio, June 1976. Proceedings.

Transportation Department, Office of University Research, Washington, D.C.