Transcription factor GATA-1 is required for erythropoiesis, yet
its full actions are unknown. We performed transcriptome analysis
of G1E-ER4 cells, a GATA-1-null erythroblast line that undergoes
synchronous erythroid maturation when GATA-1 activity is restored.
We interrogated more than 9000 transcripts at 6 time points representing
the transition from late burst forming unit-erythroid (BFU-E)
to basophilic erythroblast stages. Our findings illuminate several
new aspects of GATA-1 function. First, the large number of genes
responding quickly to restoration of GATA-1 extends the repertoire
of its potential targets. Second, many transcripts were rapidly
down-regulated, highlighting the importance of GATA-1 in gene
repression. Third, up-regulation of some known GATA-1 targets
was delayed, suggesting that auxiliary factors are required. For
example, induction of the direct GATA-1 target gene beta major
globin was late and, surprisingly, required new protein synthesis.
In contrast, the gene encoding Fog1, which cooperates with GATA-1
in beta globin transcription, was rapidly induced independently
of protein synthesis. Guided by bioinformatic analysis, we demonstrated
that selected regions of the Fog1 gene exhibit enhancer activity
and in vivo occupancy by GATA-1. These findings define a regulatory
loop for beta globin expression and, more generally, demonstrate
how transcriptome analysis can be used to generate testable hypotheses
regarding transcriptional networks.