Conformational energy calculations were carried out on penicillin α-and Β-sulfoxides and δ²- and δ³-cephalosporins, in order to identify the structural features governing their biological activity.

Results on penicillin Β-sulfoxide indicated that in its favoured conformation, the orientation of the aminoacyl group was different from the one required for biological activity. Penicillin α sulfoxide, like penicillin sulfide, favoured two conformations of nearly equal energies, but separated by a much higher energy barrier. The reduced activity of the sulfoxides despite the nonplanarity of their lactam peptide indicated that the orientations of the aminoacyl and carboxyl groups might also govern biological activity.

δ³-cephalosporins favoured two conformations of nearly equal energies, whereas δ²-cephalosporins favoured only one conformation. The lactam peptide was moderately nonplanÄr in the former, but nearly planar in the latter. The differences in the.preferred orientations of the carboxyl group between penicillins and cephalosporins were correlated with the resistance of cephalosporins to penicillinases.

We propose a molecular mechanism for the intra-cellular measurement of the ratio of the number of X chromosomes to the number of sets of autosomes, a process central to both sex determination and dosage compensation inDrosophila melanogaster. In addition to the two loci,da andSxl, which have been shown by Cline(Genetics, 90, 683, 1978)and others to be involved in these processes, we postulate two other loci, one autosomal (Ω) and the other, X-linked (π). The product of the autosomal locusda stimulates Ω and initiates synthesis of a limited quantity of repressor.Sxl and π ,both of which are X-linked, compete for this repressor as well as for RNA polymerase. It is assumed thatSxl has lower affinity than π for repressor as well as polymerase and that the binding of polymerase to one of these sites modulates the binding affinity of the other site for the enzyme. It can be shown that as a result of these postulated interactions transcription from theSxl site is proportional to the X/A ratio such that the levels ofSxl⁺ product are low in males, high in females and intermediate in the intersexes. If, as proposed by Cline, theSxl^- product is an inhibitor of X chromosome activity, this would result in dosage compensation. The model leads to the conclusion that high levels ofSxl⁺ product promote a female phenotype and low levels, a male phenotype. One interesting consequence of the assumptions on which the model is based is that the level ofSxl⁺ product in the cell, when examined as a function of increasing repressor concentration, first goes up and then decreases, yielding a bell-shaped curve. This feature of the model provides an explanation for some of the remarkable interactions among mutants at theSxl, da andmle loci and leads to several predictions. The proposed mechanism may also have relevance to certain other problems, such as size regulation during development, which seem to involve measurement of ratios at the cellular level.

Tree diameter growth is sensitive to environmental fluctuations and tropical dry forests experience high seasonal and inter-annual environmental variation. Tree growth rates in a large permanent plot at Mudumalai, southern India, were examined for the influences of rainfall and three intrinsic factors (size, species and growth form) during three 4-year intervals over the period 1988–2000.

Most trees had lowest growth during the second interval when rainfall was lowest, and skewness and kurtosis of growth distributions were reduced during this interval. Tree diameter generally explained <10% of growth variation and had less influence on growth than species identity or time interval. Intraspecific variation was high, yet species identity accounted for up to 16% of growth variation in the community. There were no consistent differences between canopy and understory tree growth rates; however, a few subgroups of species may potentially represent canopy and understory growth guilds. Environmentally-induced temporal variations in growth generally did not reduce the odds of subsequent survival.

Growth rates appear to be strongly influenced by species identity and environmental variability in the Mudumalai dry forest. Understanding and predicting vegetation dynamics in the dry tropics thus also requires information on temporal variability in local climate.