Human choline acetyltransferase gene: localization of alternative first exons

Two overlapping cosmids containing the 5' end of human choline acetyltransferase (ChAT) gene have been cloned. Using heterologous probes, we localized two alternative first exons homologous to rodent ChAT exons R and M (Misawa et al.: J Biol Chem 267:20392-20399, 1992). The sequence of rodent exon N was not conserved in the human gene. Northern blot analysis of mRNA purified from the human neuroepithelioma cell lines LA-N2 and MC-I-XC revealed that both exons R and M were transcribed in mRNA species of 6.0 and 2.5 kb. Only the 6-kb species was detected with both R- and M-specific probes in the neuroepithelioma cell line CHP126. Reverse transcription-polymerase chain reaction (RT-PCR) analysis suggested that the major mRNA species in MC-I-XC and CHP126 cells contained the proximal part of exon M spliced to exon 1, which contains the alternative ACG initiation codon. RT-PCR also allowed the characterization of a mRNA species containing exon R spliced to exon 1, but no species containing both exon R and the distal part of exon M could be detected. RT-PCR was also used to evidence an alternative exon (tentatively numbered exon 8) in the coding sequence.