The sequence specificity of alkylation for a series of benzoic acid mustard and imidazole-containing distamycin analogues: the importance of local sequence conformation.

The covalent sequence specificity of a series of nitrogen mustard and imidazole-containing analogues of distamycin was determined using modified sequencing techniques. The analogues tether benzoic acid mustard (BAM) and possess either one, two or three imidazole units. Examination of the alkylation specificity revealed that BAM produced guanine-N7 lesions in a pattern similar to conventional nitrogen mustards. The monoimidazole-BAM conjugate also produced guanine-N7 alkylation in a similar pattern to BAM, but at a 100-fold lower dose. The diimidazole and triimidazole conjugates did not produce detectable guanine-N7 alkylation but only alkylated at selected sites in the minor groove. Unexpectedly, the alkylation specificity at equivalent doses was nearly identical to that found for the previously reported pyrrole-BAM conjugates. The consensus sequence, 5'-TTTTGPuwas strongly alkylated by the triimidazole conjugate in preference to other similar sites including three occurrences of 5'-TTTTAA. Footprinting studies were carried out to examine the non-covalent DNA binding interactions. These studies revealed that the tripyrrole- BAM conjugate bound non-covalently to the same AT-rich sites as distamycin. In contrast, whereas the Im3lexitropsin bound non-covalently to GC-rich sequences, the triimidazole-BAM conjugate did not detectably footprint to either GC- or AT-rich regions at equivalent doses. The results indicate that the alkylation event is not solely dictated by the non-covalent binding and might be influenced by a unique sequence dependent conformational feature of the consensus sequence 5'-TTTTGPu.     

The effects of fire regime on legume reproduction in longleaf pine savannas: is a season selective?

The biodiversity of fire-dependent ecosystems is increasingly threatened by habitat fragmentation and fire suppression. Reducing species loss requires that salient features of natural fire regimes be incorporated into managed regimes. Lightning-season burns have been emphasized as the critical component of disturbance regimes that maintain native biodiversity within endangered longleaf pine savannas, the most diverse plant community in North America. Over evolutionary time, lightning-season fire is thought to have exerted strong selection pressure on plant pollinator interaction, flower production, and seedling establishment. In this study, season of burn had no effect on pollination activity of native legumes. Contrary to the conventional paradigm, we found a wide range of reproductive responses among dominant legumes in response to the season of burn treatments, suggesting that a variable fire season, rather than a single season of burn, is appropriate to maintain a greater variety of native species. We propose that varying the components of fire regimes, rather than selecting a particular fire regime, is likely to be important to conserve biodiversity in this and other fire-dependent communities.     

One falls,we all fall: How boys of color develop close peer mentoring relationships

The aim of this study was to investigate the processes involved in developing close peer mentoring relationships among African American and Latino male adolescents in a school-based, group peer mentoring program. Qualitative one-on-one in-depth interviews were conducted with six school staff members, who administer the program, and 26 program participants about their experiences in the mentoring program. Data analysis revealed five program processes that led to close mentoring relationships: (a) rapport-building activities, (b) safe space, (c) mutual support, (d) group identity, and (e) trust. These processes ultimately led to bonding and friendship among program members, which further solidified the previous processes. Implications and future directions are discussed.     

Prehistoric sunflower (Helianthus Annuus L.) domestication in Mexico

Early remains of Helianthus annuus L. unearthed at the San Andrés site in the Gulf Coast region of Tabasco, Mexico, constitute the earliest record of domesticated sunflower. Accelerator mass spectrometry (AMS) age determinations of a large domesticated seed and achene produced dates of 4130 ± 40 years before the present (B.P.) and 4085 ± 50 B.P., respectively. These discoveries challenge the longstanding hypothesis that sunflower was domesticated in eastern North America. Moreover, when considered with other recent discoveries on plant domestication, these data suggest a reconsideration of the idea that the eastern United States was an independent hearth for domestication.