A Single Expression Site with a Conserved Leader Sequence Regulates Variation of Expression of thePneumocystis cariniiFamily of Major Surface Glycoprotein Genes

Abstract

The major surface glycoprotein (MSG) of Pneumocystis carinii is encoded by a family of related but distinct genes distributed throughout the P. carinii genome. Previous reports of the genomic and mRNA MSG structure suggested that there was a highly conserved 5'-untranslated region and a highly variable translated region. In the current study, we demonstrate that there is a single expression site for MSG expression and that different MSG genes are located downstream of this expression site. Isolation of a genomic clone containing the putative 5'-untranslated region has demonstrated that there was a single base sequencing error in what was considered to be the untranslated region. The corrected sequence reveals an extended open reading frame encoding a constant amino-terminal leader domain, with a typical signal peptide, for the MSG protein family. Since this constant amino-terminal domain is encoded by a single copy genomic sequence, a recombination/gene conversion-mediated antigenic switching event is required to effect the known variability in expressed MSG sequences. Therefore, like some bacterial and protozoan pathogens, the opportunistic fungal pathogen P. carinii contains a constant genomic site dedicated to MSG expression and a switchable downstream region for the variable part of the MSG gene family.

Genetics of Surface Antigen Expression inPneumocystis carinii

“…carinii would be heterogeneous in structure at points downstream of the UCS and that these downstream regions might differ between populations. Mapping of restriction enzyme cleavage sites downstream of the UCS produced results consistent with this prediction (31,146). The downstream heterogeneity of the UCS locus was consistent with the previously observed presence of mRNAs encoding different MSG isoforms, yet each starting with a copy of the UCS.…”

“…The divergent sequences downstream of the UCS encoded different isoforms of MSG. These findings were confirmed by another group using a similar approach (31).…”

“…The strangeness of the UCS became more evident when it was then shown to be absent from the majority of MSG genes in the genome because the UCS hybridized to only one of the 15 chromosomes resolved by PFGE, whereas MSG gene probes hybridized to all 15 chromosomes (31,170). The restriction of the UCS to a single PFGE band suggested that it might be unique in the genome.…”

Colonization by Pneumocystis jirovecii and Its Role in Disease

“…Molecular examination of Msg genomic localization and expression revealed that the msg gene undergoes extensive genomic rearrangement, resulting in variations in its antigenic properties (148,161,203,280,281). The expressed msg gene copy is positioned at a specific expression site in the genome, with a single protein isoform expressed at one time (73,283,305). Variation of the Msg protein appears to occur through transcriptional regulation and involves complex genomic rearrangement resulting in variant expression of msg genes.…”

Telomere as an Important Player in Regulation of Microbial Pathogen Virulence

“…Transcribed MSG messengers always contained an upstream conserved sequence (UCS) (Edman et al 1996;Sunkin & Stringer 1997;Wada & Nakamura 1996;Wada et al 1995), which has only one copy in the P. carinii genome (Edman et al 1996;Wada et al 1995), suggesting that MSG is transcribed from a specific expression site marked with the unique UCS element. In addition, translation initiation codon on an MSG mRNA is located in the sequence transcribed from the UCS (Edman et al 1996;Wada et al 1995). Therefore, transcribing MSG from UCS-containing expression site is essential for proper MSG translation.…”