A novel transcript overlapping the myelin basic protein gene.

Myelin basic protein (MBP) is a major constituent of myelin synthesized by oligodendrocytes and Schwann cells. We have investigated the expression of mouse MBP RNAs outside the nervous system. Nuclease protection experiments indicate that RNAs containing exon 1 and not the six downstream exons of the MBP gene are transcribed in various hemopoietic tissues. We have isolated a hemopoietic MBP-related (HMBPR) cDNA clone from a mouse bone marrow cDNA library screened with an MBP cDNA probe. This clone contains exons 1a and 1b and a part of intron 1 of the MBP gene. An additional 5' region, encoded by at least three unidentified exons, lies upstream of exon 1a. The HMBPR clone corresponds to a 5-kb RNA expressed in bone marrow, spleen, thymus, and macrophagic cells. This transcript is expressed at a similar level in brain, although at a lower level than the classical 2-kb mRNA. These data indicate that a new transcript, overlapping the MBP transcription unit and controlled by a distinct promoter, is expressed in hemopoietic tissues. This RNA might encode a 21-kDa protein sharing a common domain with MBP.