A single base deletion in the Tfm androgen receptor gene creates a short-lived messenger RNA that directs internal translation initiation

Testosterone-resistant male mice hemizygous for the X-chromosome-linked mutant gene Tfm express detectable but severely reduced levels of androgen receptor mRNA, amounting to about 10% of the level found in normal male littermates. No structural abnormality could be identified in the coding region of the messenger by a series of RNase-protection assays. However, cell-free translation of RNAs transcribed in vitro from enzymatically amplified overlapping segments of exon 1 revealed a truncated receptor protein and helped to localize the site of premature termination. Sequence analysis of the relevant DNA segment disclosed that deletion of a single nucleotide in the hexacytidine stretch at position 1107-1112 alters the reading frame of the messenger and introduces 41 missense amino acids before a premature termination codon at position 1235-1237. Separately initiated carboxyl-terminal polypeptides are synthesized in vitro, starting probably at the in-frame AUG codon 1507-1509, which lies in a favorable context for translation initiation, and at the non-AUG codon 1144-1146. Transcriptional impairments of the Tfm gene were ruled out by a quantitative analysis of enzymatically amplified nuclear RNA precursors. No other change could be identified by sequencing the complete coding region of Tfm cDNA. The finding of the unsuspected termination codon and the evidence of internally initiated carboxyl-terminal polypeptides reconcile previous conclusions and account for all known phenotypic properties of the mutation.