SpSoxB1, a maternally encoded transcription factor asymmetrically distributed among early sea urchin blastomeres

We have identified a Sox family transcription factor, SpSoxB1, that is asym metrically distributed among blastomeres of the sea urchin embryo during cl eavage, beginning at 4th cleavage, SpSoxB1 interacts with a cis element tha t is essential for transcription of SpAN, a gene that is activated cell aut onomously and expressed asymmetrically along the animal-vegetal axis, In vi tro translated SpSoxB1 forms a specific complex with this cis element whose mobility is identical to that formed by a protein in nuclear extracts, An anti-SpSoxB1 rabbit polyclonal antiserum specifically supershifts this DNA- protein complex and recognizes a single protein on immunoblots of nuclear p roteins that comigrates with in vitro translated SpSoxB1. Developmental imm unoblots of total proteins at selected early developmental stages, as well as EMSA of egg and 16-cell stage proteins, show that SpSoxB1 is present at low levels in unfertilized eggs and progressively accumulates during cleava ge, SpSoxB1 maternal transcripts are uniformly distributed in the unfertili zed egg and the protein accumulates to similar, high concentrations in all nuclei of 4- and 8-cell embryos, However, at fourth cleavage, the micromere s, which are partitioned by asymmetric division of the vegetal 4 blastomere s, have reduced nuclear levels of the protein, while high levels persist in their sister macromeres and in the mesomeres, During cleavage, the uniform maternal SpSoxB1 transcript distribution is replaced by a zygotic nonveget al pattern that reinforces the asymmetric SpSoxB1 protein distribution and reflects the corresponding domain of SpAN mRNA accumulation at early blastu la stage (similar to 150 cells). The vegetal region lacking nuclear SpSoxB1 gradually expands so that, after blastula stage, only cells in differentia ting ectoderm accumulate this protein in their nuclei, The results reported here support a model in which SpSoxB1 is a major regulator of the initial phase of asymmetric transcription of SpAN in the nonvegetal domain by virtu e of its distribution at 4th cleavage and is subsequently an important spat ial determinant of expression in the early blastula, This factor is the ear liest known spatially restricted regulator of transcription along the anima l-vegetal axis of the sea urchin embryo.