The leukemia-associated-protein (LAP) domain, a cysteine-rich motif, is present in a wide range of proteins, including MLL, AF10, and MLLT6 proteins.

We have identified and further characterized a Caenorhabditis elegans gene, CEZF, that encodes a protein with substantial homology to the zinc finger and leucine zipper motifs of the human gene products AF10, MLLT6, and BR140. The first part of the zinc finger region of CEZF has strong similarity to the corresponding regions of AF10 (66%) and MLLT6 (64%) at the cDNA level. As this region is structurally different from previously described zinc finger motifs, sequence homology searches were done. Twenty-five other proteins with a similar motif were identified. Because the functional domain of this motif is potentially disrupted in leukemia-associated chromosomal translocations, we propose the name of leukemia-associated protein (LAP) finger. On the basis of these comparisons, the LAP domain consensus sequence is Cys1-Xaa1-2-Cys2-Xaa9-21-Cys3-Xaa2-4 -Cys4-Xaa4-5-His5-Xaa2-Cys6-Xaa12-46 - Cys7-Xaa2-Cys8, where subscripted numbers represent the number of amino acid residues. We review the evidence that this motif binds zinc, is the important DNA-binding domain in this group of regulatory proteins, and may be involved in leukemogenesis.

Torsional directed walks, entropic elasticity, and DNA twist stiffness

DNA and other biopolymers differ from classical polymers because of their torsional stiffness. This property changes the statistical character of their conformations under tension from a classical random walk to a problem we call the “torsional directed walk.” Motivated by a recent experiment on single lambda-DNA molecules [Strick, T. R., Allemand, J.-F., Bensimon, D., Bensimon, A. & Croquette, V. (1996) Science 271, 1835–1837], we formulate the torsional directed walk problem and solve it analytically in the appropriate force regime. Our technique affords a direct physical determination of the microscopic twist stiffness C and twist-stretch coupling D relevant for DNA functionality. The theory quantitatively fits existing experimental data for relative extension as a function of overtwist over a wide range of applied force; fitting to the experimental data yields the numerical values C = 120 nm and D = 50 nm. Future experiments will refine these values. We also predict that the phenomenon of reduction of effective twist stiffness by bend fluctuations should be testable in future single-molecule experiments, and we give its analytic form.

X-ray emission from clusters and groups of galaxies

Recent major advances in x-ray imaging and spectroscopy of clusters have allowed the determination of their mass and mass profile out to ≈1/2 the virial radius. In rich clusters, most of the baryonic mass is in the gas phase, and the ratio of mass in gas/stars varies by a factor of 2–4. The baryonic fractions vary by a factor of ≈3 from cluster to cluster and almost always exceed 0.09 h50−[3/2] and thus are in fundamental conflict with the assumption of Ω = 1 and the results of big bang nucleosynthesis. The derived Fe abundances are 0.2–0.45 solar, and the abundances of O and Si for low redshift systems are 0.6–1.0 solar. This distribution is consistent with an origin in pure type II supernova. The amount of light and energy produced by these supernovae is very large, indicating their importance in influencing the formation of clusters and galaxies. The lack of evolution of Fe to a redshift of z ≈ 0.4 argues for very early enrichment of the cluster gas. Groups show a wide range of abundances, 0.1–0.5 solar. The results of an x-ray survey indicate that the contribution of groups to the mass density of the universe is likely to be larger than 0.1 h50−2. Many of the very poor groups have large x-ray halos and are filled with small galaxies whose velocity dispersion is a good match to the x-ray temperatures.