The transcription factor Pax5 is essential for initiating B cell lineage commitment, but its role in maintaining commitment is unknown. Using conditional Pax5 inactivation in committed pro-B cells, we demonstrate that Pax5 is required not only to initiate its B lymphoid transcription program, but also to maintain it in early B cell development. As a consequence of Pax5 inactivation, previously committed pro-B cells regained the capacity to differentiate into macrophages in vitro and to reconstitute T cell development in vivo in RAG2-/- mice. Hence, Pax5 expression is continuously required to maintain B cell lineage commitment, because its loss converts committed pro-B cells into hematopoietic progenitors with multilineage potential.

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The transcription factor Pax6 is required for normal development of the central nervous system, the eyes, nose, and pancreas. Here we show that the transactivation domain (TAD) of zebrafish Pax6 is phosphorylated in vitro by the mitogen-activated protein kinases (MAPKs) extracellular-signal regulated kinase (ERK) and p38 kinase but not by Jun N-terminal kinase (JNK). Three of four putative proline-dependent kinase phosphorylation sites are phosphorylated in vitro. Pax6 is a member of the paired box-containing Pax 1 gene family of transcription factors containing nine human (PAX1-PAX9) and murine (Pax1-Pax9) family members (1). The paired box was first discovered in Drosophila as encoding a conserved 125-128 amino acid paired domain unique to this family of developmental control genes (2). Pax6 was initially cloned from human (3), mouse (4), zebrafish (5), and quail (6). Subsequently, the Drosophila eyeless gene was shown to be a Pax6 homolog and Pax6 homologs have now been described in other invertebrates such as flatworm, ribbonworm, Caenorhabditis elegans, squid, sea urchin, and ascidian (reviewed in Ref. 7) as well as in amphioxus (8). Pax6 is expressed in the developing central nervous system, the eyes, nose, and pancreas in higher vertebrates (4, 5, 9, 10) and plays a pivotal role in the development of these organs (7,11,12). Loss of a functional Pax6 allele results in the Mendelian syndromes aniridia, Peter's anomaly, and congenital cataracts in man (13) and Small eye in rodents (14). Pax6 acts high up in the regulatory hierarchy controlling eye development in both vertebrates and invertebrates (reviewed in Ref. 15). Eyeless, ribbonworm-, squid-, ascidian-, zebrafish-, or mouse Pax6 are all able to induce ectopic eyes in Drosophila upon targeting their expression to different imaginal discs (16 -20). We have recently found that zebrafish contain two Pax6 genes, Pax6.1 and Pax6.2, which are expressed in both overlapping and distinct regions during development of the eyes and the central nervous system. Both these genes are able to induce ectopic eyes in Drosophila (18).The paired domain is a bipartite DNA-binding domain containing an N-and a C-terminal subdomain each with a helixturn-helix motif (21, 22). Pax6, like Pax3, Pax4, and Pax7, also harbors a second DNA-binding domain, the paired-type homeodomain (2, 7, 23). In Pax6 this domain is separated from the N-terminal located paired domain by a flexible, acidic linker region (5, 7). The region C-terminal to the homeodomain is enriched in proline, serine, and threonine residues (PST-rich) and acts as a transactivation domain (TAD) (24 -27).It has previously been shown that quail Pax6 proteins expressed in the neuroretina are phosphoproteins and phosphoamino acid analysis revealed phosphoserine and a minor proportion of phosphothreonine (28). The activity of many transcription factors is regulated in a rapid and reversible manner by specific phosphorylation events mediated by protein kinases acting in signaling cascades initiated by extracellular stimuli (r...

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