Expression of the human carbonic anhydrase I gene is activated late in fetal erythroid development and regulated by stage-specific trans-acting factors.

Using flow cytometric analysis of red cells from different stages of ontogeny with anti-CAI antibody, it was shown that the human carbonic anhydrase I (HCAI) gene product appears in a developmental stage-specific manner. Virtually no CAI protein was detectable in fetal red cells prior to birth. However, at about the time of normal delivery (40 weeks gestation) CAI production is switched on. The proportion of cells containing CAI reaches adult levels during the second half of the first year of life. Northern analysis suggests that the appearance of CAI protein results directly from the activation of the gene and the production of new mRNA. A transient heterokaryon system was set up by fusing the erythroleukaemic cell lines MEL C88 (a mouse cell line in which CAI is expressed) and K562 SAI (a human cell line with an embryonic/fetal phenotype, not expressing CAI). SP6 RNAase mapping of RNA from the fused cells showed activation of the human CAI gene. This shows the developmental stage-specific expression of HCAI to be regulated by trans-acting factors.