H1FOO is coupled to the initiation of oocytic growth.

We previously reported the discovery of a novel mammalian H1 linker histone termed H1FOO (formerly H1OO), a replacement H1, the expression of which is restricted to the growing/ maturing oocyte and to the zygote. The significance of this pre-embryonic H1 draws on its substantial orthologous conservation, singular structural attributes, selectivity for the germ cell lineage, prolonged nucleosomal residence, and apparent predominance among germ cell H1s. Herein, we report that the intronic, single-copy, five-exon (> or =5301 base pair) H1foo gene maps to chromosome 6 and that the corresponding primary H1foo transcript gives rise to two distinct, alternatively spliced mRNA species (H1foo(alpha) and H1foo(beta)). The expression of the oocytic H1FOO transcript and protein proved temporally coupled to the recruitment of resting primordial follicles into a developing primary follicular cohort and thus to the critical transition marking the onset of oocytic growth. The corresponding potential protein isoforms (H1FOO(alpha) and H1FOO(beta)), both nuclear localization sequence-endowed but export consensus sequence-free and possessing a significant net positive charge, localized primarily to perinucleolar heterochromatin in the oocytic germinal vesicle. Further investigation will be required to define the functional role of the H1FOO protein in the ordering of the chromatin of early mammalian development as well as its potential role in defining the primordial-to-primary follicle transition.