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The x-ray analysis of the monoclinic form of yeast tRNA~h. has been taken to a resolution of 2.5 A by the method of isomorphous replacement. The model ropsed at 3 A has been confirmed and extended to revea a ditional features of the tertiary structure and of the stereochemistry. An extensive hydrogen bonding network is described involving specific interactions between bases and the ribose-phosphate backbone. The structure of a -U base pair has ben solved.In this paper we present the second stage of the x-ray analysis of the monoclinic form of yeast phenylalanine tRNA. A year ago we proposed a molecular model based on the interpretation of an electron density map at 3 A resolution calculated with phases obtained from isomorphous replacement (1). A similar model was also proposed by Kim et al. (2) for the closely related orthorhombic form. Our map was of sufficient quality to trace the ribose-phosphate chain and to assign all except five nucleotides to peaks in the electron density. These nucleotides lay in a corner of the molecule where the density was strong, but could not be interpreted with as much confidence as the rest of the structure because of tight intra-and intermolecular packing, and two alternative chain tracings were given. In the rest of the molecule we were able to define unambiguously a number of base-base interactions involved in maintaining the tertiary structure, and to describe the extensive stacking interactions present. Many of these interactions could be related to invariances or semiinvariances in the generalized nucleotide sequence of tRNA ( Fig. 1). We showed how other species of tRNA could be accommodated in the same tertiary structure by coordinated changes of sequence (1, 3). The results of a companion study of the chemical reactivity of yeast tRNAPhe were found to be in good accord with the model (4, 5).The x-ray analysis has now been extended to 2.5 A by the method of isomorphous replacement using six heavy atom derivatives to produce an improved electron density map. The structure reported previously (1) has been confirmed and the tracing of the chain in the uncertain region is now unambiguous. The crystallographic discrepancy index, R, has been reduced from 46 to 39%, a value as good as that for protein structure determinations at a comparable stage. The atomic coordinates are unlikely to suffer any large changes in subsequent refinement, and are therefore being published (8).The 2.5 A model allows us to complete the detailed description of base-base interactions in the molecule (Fig. 2). It has also revealed a large number of base-backbone interactions and a new semi-invariant base pair. A hydrogen-bonding network runs through most of the molecule, apart from the exposed amino-acid and anticodon stems. A number of the ribose groups in the nonhelical regions are of the 2'-endo type rather than the 3'-endo found in RNA helices. We now have a picture of the ordered complexity of a folded RNA molecule, a complexity as great as that of a protein. An interesting by-product is...

How do undergraduates engage authority figures in college? Existing explanations predict class-based engagement strategies. Using in-depth interviews with 89 undergraduates at an elite university, I show how undergraduates with disparate precollege experiences differ in their orientations toward and strategies for engaging authority figures in college. Middle-class undergraduates report being at ease in interacting with authority figures and are proactive in doing so. Lower-income undergraduates, however, are split. The privileged poor—lower-income undergraduates who attended boarding, day, and preparatory high schools—enter college primed to engage professors and are proactive in doing so. By contrast, the doubly disadvantaged—lower-income undergraduates who remained tied to their home communities and attended local, typically distressed high schools—are more resistant to engaging authority figures in college and tend to withdraw from them. Through documenting the heterogeneity among lower-income undergraduates, I show how static understandings of individuals’ cultural endowments derived solely from family background homogenize the experiences of lower-income undergraduates. In so doing, I shed new light on the cultural underpinnings of education processes in higher education and extend previous analyses of how informal university practices exacerbate class differences among undergraduates.

Existing explanations of class marginality predict similar social experiences for all lower‐income undergraduates. This article extends this literature by presenting data highlighting the cultural and social contingencies that account for differences in experiences of class marginality. The degree of cultural and social dissimilarity between one's life before and during college helps explain variation in experiences. I contrast the experiences of two groups of lower‐income, black undergraduates—the Doubly Disadvantaged and Privileged Poor. Although from comparable disadvantaged households and neighborhoods, they travel along divergent paths to college. Unlike the Doubly Disadvantaged, whose precollege experiences are localized, the Privileged Poor cross social boundaries for school. In college, the Doubly Disadvantaged report negative interactions with peers and professors and adopt isolationist strategies, while the Privileged Poor generally report positive interactions and adopt integrationist strategies. In addition to extending present conceptualizations of class marginality, this study advances our understanding of how and when class and culture matter in stratification processes in college.