The vertebrate hindbrain is divided early in embryogenesis along its anterior-posterior axis into eight segments known as rhombomeres. This provides an excellent model for studying early segmentation and region-specific transcriptional domains. MafB, a basic domain leucine zipper transcription factor, is the first gene known to be expressed in the presumptive rhombomere 5 and 6 domain (r5-r6). MafB expression is directly activated by the homeodomain protein vHnf1. vHnf1 and MafB share an anterior expression limit at the r4/r5 boundary but vHnf1 expression extends beyond the posterior limit of MafB and, therefore, cannot establish the posterior expression boundary of MafB. Through the use of in situ hybridization, immunofluorescence, and chromatin immunoprecipitation analyses, I have determined that the caudal-related homeodomain protein Cdx1 establishes the posterior boundary of MafB by directly inhibiting expression beyond the r6/r7 boundary. My results indicate that MafB is one of the earliest direct targets of Cdx1.

The vertebrate hindbrain is divided early in embryogenesis along its anterior-posterior axis into eight segments known as rhombomeres. This provides an excellent model for studying early segmentation and region-specific transcriptional domains. MafB, a basic domain leucine zipper transcription factor, is the first gene known to be expressed in the presumptive rhombomere 5 and 6 domain (r5-r6). MafB expression is directly activated by the homeodomain protein vHnf1. vHnf1 and MafB share an anterior expression limit at the r4/r5 boundary but vHnf1 expression extends beyond the posterior limit of MafB and, therefore, cannot establish the posterior expression boundary of MafB. Through the use of in situ hybridization, immunofluorescence, and chromatin immunoprecipitation analyses, I have determined that the caudal-related homeodomain protein Cdx1 establishes the posterior boundary of MafB by directly inhibiting expression beyond the r6/r7 boundary. My results indicate that MafB is one of the earliest direct targets of Cdx1.