Nucleosome organization during germ cell development in the sea cucumber Holothuria tubulosa

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Abstract
Conformational changes that occur in chromatin from developing germ cells of the echinoderm H. tubulosa were probed with micrococcal nuclease. The extent of DNA degradation to acid-soluble nucleotides is highest in chromatin at the early stages of gonad growth, being drastically subdued in the mature sperm cell. Production of nucleosomal particles also varies with development, involving at least 70% of the chromatin at the final stage of maturation, whereas in immature germ cells it remains much lower. Electrophoretic analysis for DNA size showed that the average nucleosome repeat length, about 227 base pairs, does not change throughout the maturation process. Kinetics of the enzyme reaction revealed that, although brief digestion of chromatin from both immature gonads and sperm yields comparable series of higher oligomers, extensive digest patterns differ widely. Sperm chromatin, highly protected, releases a 275 base pair intermediate fragment, wholly absent in immature gonads. The 145 base pair core released in both chromatins is not further digested in sperm. Compared to sperm chromatin, immature germ cells are much more susceptible to fragmentation, yielding the usual set of smaller subnucleosomal fragments. These data suggest the induction of differential accessibilities of chromatin DNA with maturation, which is not accompanied by displacement of the histone complement. The histone variants present in this species may be instrumental in the process.