The ATP-dependent chromatin assembly factor (ACF) spaces nucleosomes to promote formation of silent chromatin. Two copies of its ATPase subunit SNF2h bind opposite sides of a nucleosome, but how these protomers avoid competition is unknown. SNF2h senses the length of DNA flanking a nucleosome via its HAND-SANT-SLIDE (HSS) domain, yet it is unclear how this interaction enhances remodeling. Using covalently connected SNF2h dimers we show that dimerization accelerates remodeling and that the HSS contributes to communication between protomers. We further identify a nucleotide-dependent conformational change in SNF2h. In one conformation the HSS binds flanking DNA, and in another conformation the HSS engages the nucleosome core. Based on these results, we propose a model in which DNA length sensing and translocation are performed by two distinct conformational states of SNF2h. Such separation of function suggests that these activities could be independently regulated to affect remodeling outcomes.

Data associated with over 9000 accidents involving large trucks and combination vehicles during a two-year period on freeways in the greater Los Angeles area are analyzed relative to collision factors, accident severity, incident duration, and lane closures. Relationships between type of collision and accident characteristics are explored using log-linear models. The results point to significant differences in several immediate consequences of truck-related freeway accidents according to collision type. These differences are associated both with the severity of the accident, in terms of injuries and fatalities, and the impact of the accident on system performance, in terms of incident duration and lane closures. Hit-object and broadside collisions are the most severe types in terms of fatalities and injuries, respectively, and single-vehicle accidents are relatively more severe than two-vehicle accidents. The durations of accident incidents are found to be log-normally distributed for homogeneous groups of truck accidents, categorized according to type of collision and, in some instances, severity. The longest durations are typically associated with overturns. 

This working paper reports initial results of a set of analyses investigating differences between non-truck involved and truck-involved crashes. Data was selected from the California Department of Transportation TASAS accident reporting system. Results indicate that there are differences in primary collision factors (as assigned by the California Highway Patrol through issuance of traffic citations) between truck-involved and non-truck-involved crashes in the vicinity of freeway interchanges. "Speeding' in truck-involved crashes is cited at approximately one-half its rate for non-truck-involved crashes. Additionally, there is no statistical difference in the appearance of the "Uninvolved Motorist" TASAS party designation between truck and non-truck-involved crashes.

In the research presented here, an attempt is made to analyze the entire daily activity pattern simultaneously in terms of its space, time, and activity category dimensions. The focus of the research is on developing procedures for the dynamic analysis of possible changes in activity patterns that may result from or lead to changes in the characteristics of the household. The intent is to trace the daily activity patterns of groups of travelers that have undergone significant change during multiple points in time, in a manner that will shed some light on both the nature and extent of associated changes in travel behavior. To check on the reliability of the modeling process and to understand its peculiarities, complementary analyses of pattern attributes, and time-series analyses of patterns were performed on the same data set. Results from these complementary analyses are briefly outlined here, and it is hoped that these results aid in placing the present approach in the perspective of the rich legacy of previous work.

The chromatin remodeling complex ACF helps establish the appropriate nucleosome spacing for generating repressed chromatin states. ACF activity is stimulated by two defining features of the nucleosomal substrate: a basic patch on the histone H4 N-terminal tail and the specific length of flanking DNA. However, the mechanisms by which these two substrate cues function in the ACF remodeling reaction is not well understood. Using electron paramagnetic resonance spectroscopy with spin-labeled ATP analogs to probe the structure of the ATP active site under physiological solution conditions, we identify a closed state of the ATP-binding pocket that correlates with ATPase activity. We find that the H4 tail promotes pocket closure. We further show that ATPase stimulation by the H4 tail does not require a specific structure connecting the H4 tail and the globular domain. In the case of many DNA helicases, closure of the ATP-binding pocket is regulated by specific DNA substrates. Pocket closure by the H4 tail may analogously provide a mechanism to directly couple substrate recognition to activity. Surprisingly, the flanking DNA, which also stimulates ATP hydrolysis, does not promote pocket closure, suggesting that the H4 tail and flanking DNA may be recognized in different reaction steps.

The SNF2h remodeler slides nucleosomes most efficiently as a dimer, yet how the two protomers avoid a tug-of-war is unclear. Furthermore, SNF2h couples histone octamer deformation to nucleosome sliding, but the underlying structural basis remains unknown. Here we present cryo-EM structures of SNF2h-nucleosome complexes with ADP-BeFx that capture two potential reaction intermediates. In one structure, histone residues near the dyad and in the H2A-H2B acidic patch, distal to the active SNF2h protomer, appear disordered. The disordered acidic patch is expected to inhibit the second SNF2h protomer, while disorder near the dyad is expected to promote DNA translocation. The other structure doesn't show octamer deformation, but surprisingly shows a 2 bp translocation. FRET studies indicate that ADP-BeFx predisposes SNF2h-nucleosome complexes for an elemental translocation step. We propose a model for allosteric control through the nucleosome, where one SNF2h protomer promotes asymmetric octamer deformation to inhibit the second protomer, while stimulating directional DNA translocation.