PDF《SPERM CHROMATIN》

216 G. Fuentes-Mascorro et al. morphological standpoint. These changes are mostly due to the loss of most (if not all) of the cytoplasm and the development of a motile tail. During these steps, and after the transcriptional activity of the genome has ceased, the sperm must develop a highly condensed nucleus with a species-specific shape, and with diminished replicative, translational, and DNA repair activi- ties. The process involves a complicated series of reactions through which somatic histones and nonhistone chromatin proteins are replaced during a variable period of time and in a species- specific way by one or more protamine types [40]. In the rat, mouse, and sheep, this replace- ment involves a set of special proteins known as transition proteins. Histone exchange by protamines constitutes one of the most remarkable changes in chromatin structure observed in eukaryotes and seems to be essential for sperm nuclear organization. Histone Replacement Protamines are highly basic proteins about half the size of a typical histone (5C8 kDa). From

55 up to 79% of the aminoacid residues of protamines are arginines, permitting a strong DNA binding. Protamines contain a significant number of cystein residues that are very impor- tant during the final stages of sperm nuclear maturation because they participate in sperm chromatin compaction by forming multiple inter- and intraprotamine disulfide cross-links [41]. All these interactions contribute to make mammalian sperm DNA the most condensed eukary- otic DNA [67]. Mature sperm chromatin is notably refractory to in situ hydrolytic enzyme treatment [1], not easily stainable by the Feulgen method, and interacts scarcely with dyes and fluorochromes [60]. This remarkable arrangement explains the resilience of sperm nuclei [38], and why isolated mammalian sperm nuclei, completely deprived of nuclear envelope, preserve their volume and shape [4]. While the histoneCDNA structure is highly conserved in most eucaryotic organism, prota- mines show a high degree of species specificity [5]. Not all species replace histones in the sperm nucleus during spermatogenesis: some species, such as carp [16], retain histones as typical proteins in the sperm nuclei. Other species, such as humans, retain up to 15% of their original histones content [22]. Birds completely replace histones in the sperm nucleus [48]. Bulls [5], rams [40], and rats [25] contain only one type of protamine, whereas mice and humans have two [5] (Table 1). Table 1. Chromatin structuration on different animal species Animal Protamine Transition species Histones type proteins Reference Rabbit P1

36 Bull P1 25,

49 Ram P1 Yes

25 Stallion P1, P2 25, 39,

63 Bull P1 25,

49 Man P1, P2 Yes Man Yes, 15%

22 Mice Yes P1, P2 Yes 6, 52,

55 Rat P1 Ye........