Residence at the expression site is necessary and sufficient for the transcription of surface antigen genes of Pneumocystis carinii.

The major surface glycoprotein (MSG) of P. carinii f. sp. carinii is a family of proteins encoded by a family of heterogeneous genes. Messenger RNAs encoding different MSG isoforms start with the same sequence, called the upstream conserved sequence (UCS), which is encoded by a single locus. The mechanism by which the UCS becomes part of different MSG mRNAs is not obvious because at least 15 loci, which are distributed throughout the genome, encode MSGs. One possibility is that attachment to the UCS locus is required for the transcription of an MSG gene. The alternative to this expression site model is that mRNAs acquire the UCS by RNA splicing. To distinguish between these two models, UCS/MSG junctions in the genome were compared with UCS/MSG junctions in mRNA. The UCS/MSG junctions in the mRNA matched those in the genome, as would be expected if splicing did not contribute to the attachment of the UCS to the 5' ends of MSG mRNAs. Given that few if any MSG mRNAs lack the UCS, the correspondence between the UCS/MSG junctions in transcripts and those in the genome indicates that attachment to the UCS is both necessary and sufficient for transcription of an MSG gene.