Amino acid sequences have been compared for thermophilic and mesophilic molecules of ferredoxin, glyceraldehyde-3-phosphate dehydrogenase, and lactate dehydrogenase. It is shown that Gly, Ser, Ser, Lys, and Asp in mesophiles are generally substituted by Ala, Ala, Thr, Arg, and Glu, respectively, in thermophiles. These exchanges suggest that thermal stability can be achieved by the addition of many small changes throughout the molecule without significant change in the backbone conformation. Their overall effect is primarily to increase internal and decrease external hydrophobicity as well as to favor helix stabilizing residues in helices. These substitutions minimize interruption of function or internal residue packing arrangements. Although the analysis has been confined to the above-mentioned molecules, the observed stabilizing principles may be more generally applicable.
Comparisons have been made between amino acid sequences of 26 chloroplast-type ferredoxins and 16 bacterial-type ferredoxins. Their structural characteristics are described and related to a three-dimensional structure of a chloroplast-type ferredoxin. Aspects of molecular evolution of these ferredoxins are presented together with a phylogenetic tree including both chloroplast- and bacterial-type ferredoxins.
Everyone who has observed and considered a field of marine algae on the seashore at low tide knows that there are four very conspicuous groups of plants in the ocean. They are the blue-green, red, brown, and green algae. The obvious question is, Why does the sea contain plants of such a variety of colors when the land supports only green vegetation?… It is here suggested that the various pigments involved were originally called into existence in response to certain physical environmental forces or variables and that by far the most important of these external factors was illumination, the quantity or quality of which was determined or modified by two different media, (a) the sea water in which the organisms lived and (b) the layer of atmosphere above it.
