The compositional distributions of high molecular weight DNA fragments from 20 species belonging to 9 out of the 17 eutherian orders were investigated by analytical CsCl density gradient centrifugation and by preparative fractionation in Cs2SO4/BAMD density gradients followed by analysis of the fractions in CsCl. These compositional distributions reflect those of the isochores making up the corresponding genomes. A "general distribution" was found in species belonging to eight mammalian orders. A "myomorph distribution" was found in Myomorpha, but not in the other rodent infraorders Sciuromorpha and Histricomorpha, which share the general distribution. Two other distributions were found in a megachiropteran (but not in microchiropteran, which, again, shares the general distribution) and in pangolin (a species from the only genus of the order Pholidota), respectively. The main difference between the general distribution and all other distributions is that the former contains sizable amounts (6-10%) of GC-rich isochores (detected as DNA fragments equal to, or higher than, 1.710 g/cm3 in modal buoyant density), which are scarce, or absent, in the other distributions. This difference is remarkable because gene concentrations in mammalian genomes are paralleled by GC levels, the highest gene concentrations being present in the GC-richest isochores. The compositional distributions of mammalian genomes reported here shed light on mammalian phylogeny. Indeed, all orders investigated, with the exception of Pholidota, seem to share a common ancestor. The compositional patterns of the megachiropteran and of Myomorpha may be derived from the general pattern or have independent origins.

The compositional distributions of large (main-band) DNA fragments from eight birds belonging to eight different orders (including both paleognathous and neognathous species) are very broad and extremely close to each other. These findings, which are paralleled by the compositional similarity of homologous coding sequences and their codon positions, support the idea that birds are a monophyletic group.The compositional distribution of third-codon positions of genes from chicken, the only avian species for which a relatively large number of coding sequences is known, is very broad and bimodal, the minor GC-richer peak reaching 100% GC. The very high compositional heterogeneity of avian genomes is accompanied (as in the case of mammalian genomes) by a very high speciation rate compared to cold-blooded vertebrates which are characterized by genomes that are much less heterogeneous. The higher GC levels attained by avian compared to mammalian genomes might be correlated with the higher body temperature (41--43°C) of birds compared to mammals (37°C).A comparison of GC levels of coding sequences and codon positions from man and chicken revealed very close average GC levels and standard devia-Present address: Facult6 de Mrdecine Nord, Boulevard Pierre Dramard, 13326 Marseille Cedex 15, France Correspondence to: G. Bernarditions. Homologous coding sequences and codon positions from man and chicken showed a surprisingly high degree of compositional similarity which was, however, higher for GC-poor than for GC-rich sequences. This indicates that GC-poor isochores of warm-blooded vertebrates reflect the composition of the isochores of the genome of the common reptilian ancestor of mammals and birds, which underwent only a small compositional change at the transition from cold-to warm-blooded vertebrates. In contrast, the GC-rich isochores of birds and mammals are the result of large compositional changes at the same evolutionary transition, where were in part different in the two classes of warm-blooded vertebrates.

No abstract

A deoxyribonuclease has been isolated from the testes and deferent ducts of the crab Cancer pagurus. The purification procedure involves ammonium sulfate precipitation, gel filtration on Sephadex G-200, affinity chromatography on RNA core-Sepharose, and hydroxylapatite chromatography. The enzyme shows endonucleolytic activity and is obtained free of phosphomonoesterase, phosphodiesterase, exonuclease, and RNase activities. The enzyme requires divalent cations and has

The recently isolated neutral deoxyribonuclease from crab (Cancer pagurus) testes has been characterized in its mode of action and its specificity. The enzyme is a typical endonuclease, forming 5'-phosphate oligonucleotides of large average size; after extensive digestion of calf thymus DNA over 75% of the fragments have a size larger than pentanucleotides and mononucleotides are absent. As far as specificity is concerned, thymidine is very abundant in the 5'-penultimate position ( x 50 %) and in the 3'-terminal position (37 %) and almost absent in the 5'-terminal position (z 1 %), the values quoted concerning Escherichiu coli digests of average size (P,,) between 50 and 10.Recent results on the specificity of deoxyribonucleases (DNAases) and on their use in assessing the frequency of the short nucleotide sequences recognized and split by these enzymes (see [l] for a brief review) encouraged us to screen a number of deoxyribonucleases for specificity. In the present work, we have investigated the specificity of a neutral DNAase recently isolated from Cancer pagurus testes [2]. A new procedure of preparation of the enzyme, based on the method originally developed for spleen acid DNAase [3], has been set up. The specificity of the enzyme has been determined using the labelling and separation approaches recently established for the 3'-terminal nucleotides and the 5'-terminal dinucleotides [4,5] of the oligonucleotides formed by the crab DNAase. The 5'-terminal dinucleotides released from Escherichiu coli DNA (51 "/, dG + dC) by the enzyme are very characteristic in that thymidine is almost absent from the terminal position (z 1 %) and predominates on the penultimate position (z 50 %) ; thymidine also is the most frequent (37 %) 3'-terminal nucleotide liberated by the enzyme. IS, 203-208 (1970). MATERIALS AND METHODSThe DNA preparations used in the present work were described elsewhere [6,7].Crab DNAase digestions were carried out in 0.005 M Tris-HC1 pH 7.2, 0.01 M MgC12. DNA concentration and incubation temperature are indicated in the figure legends. Activity units are defined elsewhere [3].The enzyme was inactivated by shaking the incubation mixture with 0.1 volume of chloroform-isoamyl alcohol (24: 1 ; v/v) on a Whirlimixer (Springfield, Mass.). Digests were extensively dialyzed against distilled water, except where otherwise stated. For the determination of 5'-terminal dinucleotides, they were made 0.05 M in ammonium acetate buffer pH 4.6, and dephosphorylated by incubation at 37 "C for 4 h with 0.005 units of acid phosphatase B per AZ6,, unit of digest; acid phosphatase was inactivated by shaking with chloroform -isoamyl alcohol, as indicated above.DEAE-cellulose -urea column chromatography was done as previously described [6], on undialyzed digests showing a hyperchromic shift of 34 %.The methods for the determination of the 3' and 5'-terminal nucleotides and the 5'-terminal dinucleotides have been described elsewhere [4,8,5] along with the enzyme preparations used.The statistical evaluation of data was done...