Hemoglobin Long Island is caused by a single mutation (adenine to cytosine) resulting in a failure to cleave amino-terminal methionine.

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Abstract
Hemoglobin Long Island has two separate amino acid abnormalities of .beta.-globin structure: an extension of the NH2 terminus by a methionine residue and a histidine-to-proline substitution at the normal second position. The NH2-terminal methionine residue, the translation product of an AUG initiation codon, is present only transiently in nascent proteins. Because of the general biological implications of this abnormality, we investigated the nature of the genetic defect of this mutant. We determined the sequence of the relevant portion of the .beta.-globin mRNA by means of dideoxynucleotide chain termination of the complementary DNA (cDNA) in which an oligonucleotide complementary to codons 10-17 was used as a primer for reverse transcriptase. A histidine-to-proline substitution was confirmed in the mutant mRNA by identifying an adenine-to-cytosine transversion in the second codon. However, we were unable to find any other abnormality at either the AUG initiation codon or in the 56 bases upstream from the adenine-to-cytosine transversion (encompassing most of the 5'' untranslated region of the mutant .beta.-globin mRNA). Thus, it appears that this single lesion probably interferes with the poorly understood methionine-cleaving mechanism that modulates most of prokaryotic and eukaryotic proteins.