Leubner-Metzger G, Horwitz B A, Yoder O C, Turgeon B G
Friedrich Miescher Institute, Basel, Switzerland.
Mol Gen Genet. 1997 Nov;256(6):661-73. doi: 10.1007/s004380050614.
The single mating type locus (MAT) of the heterothallic ascomycete Cochliobolus heterostrophus is composed of a pair of unlike sequences called idiomorphs, each of which encodes one MAT-specific gene (MAT-1 and MAT-2). MAT transcripts were observed in blots of poly(A)+ RNA isolated from cultures grown in minimal medium, but were not detectable after growth of the fungus in complete medium, suggesting that transcription of MAT is tightly regulated. The idiomorphs (MAT-1 = 1297-bp, MAT-2 = 1171-bp) encode transcripts of 2.2 kb (MAT-1) and 2.1 kb (MAT-2), which start 5' and end 3' of the idiomorph within sequences common to both mating types. Analyses of MAT-1 and MAT-2 cDNAs revealed obligatory splicing of one intron (55-bp in MAT-1, 52-bp in MAT-2) within each MAT-specific ORF and optional splicing of two introns (63 and 79-bp) in the long (approximately 0.55 kb) 5' untranslated leader sequences; the 3' untranslated region is 0.46 kb long. Transcription start sites were found 5' of, and within, the 79-bp intron. Optional splicing of the upstream introns and at least two transcription start sites result in three types of transcript: Type I with both 5' introns spliced, Type II with only the 63-bp intron spliced, and Type III with neither 5' intron spliced. The three transcript types are distinguished by various combinations of four short ORFs encoded by the corresponding genomic DNA, in the leader sequences of the MAT mRNAs. The transcript structure suggests several mechanisms by which expression of the MAT genes might be regulated at the level of translation during sexual development.
异宗配合子囊菌玉米小斑病菌（Cochliobolus heterostrophus）的单一交配型位点（MAT）由一对不同的序列组成，称为特异型，每个特异型编码一个MAT特异性基因（MAT-1和MAT-2）。在从基本培养基中生长的培养物中分离的聚腺苷酸加尾RNA（poly(A)+ RNA）印迹中观察到MAT转录本，但在真菌在完全培养基中生长后无法检测到，这表明MAT的转录受到严格调控。特异型（MAT-1 = 1297 bp，MAT-2 = 1171 bp）编码2.2 kb（MAT-1）和2.1 kb（MAT-2）的转录本，它们在两种交配型共有的序列中起始于特异型的5'端并终止于3'端。对MAT-1和MAT-2 cDNA的分析表明，每个MAT特异性开放阅读框（ORF）内有一个内含子（MAT-1中为55 bp，MAT-2中为52 bp）的强制剪接，以及长约0.55 kb的5'非翻译前导序列中有两个内含子（63和79 bp）的选择性剪接；3'非翻译区长度为0.46 kb。转录起始位点位于79 bp内含子的5'端及内部。上游内含子的选择性剪接和至少两个转录起始位点导致三种转录本类型：I型两个5'内含子均被剪接，II型仅63 bp内含子被剪接，III型两个5'内含子均未被剪接。这三种转录本类型通过MAT mRNA前导序列中相应基因组DNA编码的四个短ORF的不同组合来区分。转录本结构提示了几种在有性发育过程中MAT基因表达可能在翻译水平受到调控的机制。
