EXPRESSION OF FERROCHELATASE MESSENGER-RNA IN ERYTHROID AND NONERYTHROID CELLS

Ferrochelatase, which catalyses the last step in haem biosynthesis, i. e. the insertion of Fe(II) into protophorphyrin IX, is present in all cells, but is particularly abundant in erythroid cells during haemoglo binization. Using mouse ferrochelatase cDNA as a probe two ferrochelat ase transcripts, having lengths of 2.9 kb and 2.2 kb, were found in ex tracts of mouse liver, kidney, brain, muscle and spleen, the 2.9 kb tr anscript being more abundant in the non-erythroid tissues and the 2.2 kb transcript more predominant in spleen. In mouse erythroleukemia cel ls the 2.9 kb ferrochelatase transcript is also more abundant; however , following induction of erythroid differentiation by dimethyl sulphox ide there is a preferential increase in the 2.2 kb transcript, which e ventually predominates. With mouse reticulocytes, the purest immature erythroid cell population available, over 90% of the total ferrochelat ase mRNA is present as the 2.2 kb transcript. Since there is probably only one mouse ferrochelatase gene, the occurrence of two ferrochelata se transcripts could arise from the use of two putative polyadenylatio n signals in the 3' region of ferrochelatase DNA. This possibility was explored by using a 389 bp DNA fragment produced by PCR with syntheti c oligoprimers having sequence similarity with a region between the po lyadenylation sites. This fragment hybridized only to the 2.9 kb ferro chelatase transcript, indicating that the two transcripts differ at th eir 3' ends and suggesting that the 2.2 kb transcript results from the utilization of the upstream polyadenylation signal. The preferential utilization of the upstream polyadenylation signal may be an erythroid -specific characteristic of ferrochelatase gene expression.