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While microarray signal intensities are not strictly quantitative, they correlate generally with other measures of transcript abundance 1. The most abundant transcripts in G1E-ER4 cells are expected to reflect both general and erythroid-specific aspects of cellular function. We considered 96 genes with a minimum signal level of 7000 in at least one point to be abundant (Table). About half of these transcripts encode ribosomal proteins, while others function in energy metabolism, cytoskeleton, protein folding and degradation, antioxidant functions, and G-protein signaling. Abundant erythroid-specific transcripts included alpha and beta globins, ferritin heavy and light chains and carbonic anhydrase 2. The GATA-1 cofactor Fog-1 (Zfpm1) 2,3 was the only transcription factor represented in this group (Table). Like GATA-1, Fog-1 is critical to development of the megakaryocytic and erythroid lineages. As most erythroid functions of GATA-1 are believed to be Fog-1-dependent 3-5, the high-level expression of Fog-1 indicated here is in accord with high dosage requirements for GATA-1 in erythropoiesis 6,7.
Several abundant transcripts identified in G1E-ER4 cells were represented in a previously reported subtractive hybridization analysis to identify human erythroid-enriched transcripts 8 (denoted by asterisks in Table); cross-comparison of these two datasets identifies genes that are expressed selectively in erythroid cells at particularly high levels. Inclusion of transcripts related to protein translation and folding such Eef1a1, ribosomal proteins, Nsep1/YB-1 and Hsp90/Hsp86 may reflect generally high requirements for protein synthesis related to hemoglobin production. In addition, some of these proteins could have erythroid-specific roles. For example Nsep1/YB-1, a ribonucleoprotein that regulates protein synthesis through several potential mechanisms, was recently reported to augment ferritin translation by sequestering the translational inhibitor, IRP-2 under high iron conditions 9. Nsep1/YB-1 gene transcription is believed to be regulated directly by GATA-1 and the related protein GATA-2 10, which is expressed at high levels in G1E-ER4 cells prior to the induction of GATA-1 11.
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Table. Most abundant genes during G1E-ER4 cell differentiation. Genes with absolute signal intensities greater than 7000 at one or more timepoints are indicated and grouped by functional categories. The minimum and maximum absolute signal values during the time course (average of triplicates) are shown in parentheses, with arrows indicating the temporal progression. Asterisks denote human erythroid-enriched transcripts identified by Gubin et al. 8
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