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. Messen ger RNAs encoding different MSG isoforms start with the same sequence, called the upstream conserved sequence (UCS), which is encoded by a s ingle locus. The mechanism by which the UCS becomes part of different MSG mRNAs is not obvious because at least 15 loci, which are distribut ed throughout the genome, encode MSGs. One possibility is that attachm ent to the UCS locus is required for the transcription of an MSG gene. The alternative to this expression site model is that mRNAs acquire t he UCS by RNA splicing. To distinguish between these two models, UCS/M SG junctions in the genome were compared with UCS/MSG junctions in mRN A. The UCS/MSG junctions in the mRNA matched those in the genome, as w ould be expected if splicing did not contribute to the attachment of t he UCS to the 5' ends of MSG mRNAs. Given that few if any MSG mRNAs ra ck the UCS, the correspondence between the UCS/MSG junctions in transc ripts and those in the genome indicates that attachment to the UCS is both necessary and sufficient for transcription of an MSG gene.