Transcript elongation is a critical step in the production of mature messenger RNAs. Many factors have been identified that are required for transcript elongation, including Spt 5. Studies in yeast determined that spt 5 is required for cell viability, and analyses in Drosophila indicate Spt 5 is localized to sites of active transcription, suggesting it is required generally for transcription. However, the requirement for spt 5 for cell viability in a metazoan organism has not been addressed. We determined that zebrafish foggy/spt 5 is required cell-autonomously for the posterior migration of facial branchiomotor neurons from rhombomere 4 (r4) into r6 and r7 of the hindbrain. These genetic mosaics also give us the unique opportunity to determine whether spt 5 is required for mRNA transcription equivalently at all loci by addressing two processes within the same cell-neuronal migration and cell viability. In a wild-type host, spt 5 null facial branchiomotor neurons survive to at least 5 days postfertilization while failing to migrate posteriorly. This finding indicates that spt 5-dependent transcript elongation is required cell-autonomously for a complex cell migration but not for the survival of these same cells. This work provides evidence that transcript elongation is not a global mechanism equivalently required by all loci and may actually be under more strict developmental regulation.
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