X-ray structures of two single-residue mutants of DNase I: H134Q and Y76A

The structures of the single-residue mutants H134Q and Y76A of bovine pancreatic DNase I have been determined and refined including data to 2.3 and 2.4 A resolution respectively, by X-ray crystallography. H134 is an essential catalytic residue, while Y76 contributes to the binding of DNA by providing a large van der Waals contact area that stabilizes the wide minor groove seen in DNase I-DNA complexes. The mutant proteins, which show strongly reduced activities of 0.001% (H134Q) and 0.3% (Y76A), were expressed in E. coli and both crystallize in space-group C2 with almost identical unit cells. The crystal packing scheme is different from that found in wild type crystals grown under very similar conditions, presumably due to the absence of the carbohydrate moiety. In both mutants the conformation of the protein is nearly identical to that of the wild type enzyme and changes are confined to surface loops involved in packing. The disruption of the hydrogen bonds between H134, E78 and Y76 in both mutants leads to an increased mobility and positional shifts in the DNA-binding loop, mainly around residue Y76. This in turn may further reduce DNA-binding affinity and, thus, contribute to the low activity. In contrast, symmetry contacts involving residues 97-108 lead to a stabilization of the flexible loop compared to wild type DNase I.

Bone core biopsies. Plastic or paraffin?

Plastic embedding of bone core biopsy specimens has been promoted as providing superior morphology, primarily because semi-thin sections can thereby be cut at 1-2 mu. The major disadvantages of plastic embedding are that it increases the technical load, is more expensive, and potentially has its own intrinsic problems, including difficulties in performing special stains and immunoperoxidase studies. In order to investigate the possibility that semi-thin paraffin sections may provide similar morphological results without the intrinsic disadvantages of plastic sections, we examined 45 bone core biopsy specimens that were sufficiently large to process one half in plastic and the other half in paraffin following decalcification. Both were cut at 1-2 mu. Although many plastic sections appear esthetically more pleasing, semi-thin paraffin sections of very high quality can also be obtained routinely. Additional advantages of paraffin sections were the ability to perform peroxidase studies, lower cost, less technologist time, and avoidance of problems occasionally arising with plastic, such as difficulties with impregnation or problems with polymerization. Peroxidase studies were particularly useful in patients with possible myeloma that was not overt on hematoxylin-and-eosin section and in confirming the presence or source of metastatic carcinoma. We therefore recommend the use of semi-thin (1-2 mu) paraffin sections for routine examination of bone core biopsy specimens.

DNA packaging in mouse spermatids. Synthesis of protamine variants and four transition proteins

A comparison of the protein compositions of mouse late-step spermatids and cauda epididymal sperm has revealed that the relative distribution of the two amino acid sequence variants of mouse protamine differ markedly in spermatids and sperm. Sonication-resistant spermatids contain the two variants in a ratio of 1:1, while the ratio of these two proteins in cauda epididymal sperm is approx. 2:1. Labeling studies in vivo have shown that this difference is due, in part, to an asynchrony in the time of synthesis of the two protamine variants. Both proteins are synthesized in late-step spermatids, but synthesis of the tyrosine variant in sperm chromatin begins approximately one day before synthesis of the more predominant histidine variant. Analyses of the time of synthesis of protamine and the four transition proteins in late-step spermatids allowed us to estimate the spermatid stage in which these proteins are deposited on DNA and relate these events to the onset of sonication resistance in maturing spermatids. These results indicate that: (1) synthesis and deposition of protamine begins coincident with the onset of sonication resistance in early step 12 spermatids; (2) protamine deposition is complete by mid-step 15; and (3) synthesis of the transition proteins occurs coincident with protamine synthesis.