Corradi Nicolas, Gangaeva Anna, Keeling Patrick J
Canadian Institute for Advanced Research, Department of Botany, University of British Columbia, 3529-6270 University Boulevard, Vancouver, Canada.
Genomics. 2008 Apr;91(4):388-93. doi: 10.1016/j.ygeno.2007.12.006. Epub 2008 Feb 20.
Microsporidia are highly adapted parasites related to fungi with compact, gene-dense genomes. It has previously been shown in the microsporidian Antonospora locustae that transcripts from any given gene overlap with adjacent genes at a high frequency, perhaps due to the compact nature of its genome. However, it is still not known if this phenomenon is widespread among microsporidia or conserved between species, or even whether it is strictly correlated with compaction. To address these questions, we performed a comparison of transcription profiles in two microsporidian species, A. locustae and Encephalitozoon cuniculi. Transcription overlap was characterized at many A. locustae loci representing a range of gene densities, to determine if overlapping transcription correlates with the length of intergenic spacers. In parallel, we examined the first cases of transcription overlap in E. cuniculi. Using regions of the genome where the order of genes is conserved between A. locustae and E. cuniculi, we identified the transcriptional processing points in both species to determine how the process changes through evolutionary time. We show that there is little conservation of processing points between species and indeed that the process differs in important ways in the two genomes. Overall, A. locustae transcripts generally start just upstream of the start codon, but terminate well within or beyond downstream genes. In contrast, E. cuniculi transcripts often initiate within upstream genes, but more frequently terminate prior to the downstream gene. This process appears to have predictable characteristics within a given genome, but to be relatively flexible between species, presenting further challenges to the study of gene expression in these obligately intracellular parasites.
微孢子虫是与真菌相关的高度适应性寄生虫，具有紧凑、基因密集的基因组。先前在微孢子虫蝗虫微孢子虫中已经表明，任何给定基因的转录本与相邻基因的重叠频率很高，这可能是由于其基因组的紧凑性质。然而，目前尚不清楚这种现象在微孢子虫中是否普遍存在，或者在物种之间是否保守，甚至它是否与基因组紧凑严格相关。为了解决这些问题，我们对两种微孢子虫物种，即蝗虫微孢子虫和兔脑炎微孢子虫的转录谱进行了比较。在许多代表一系列基因密度的蝗虫微孢子虫基因座上对转录重叠进行了表征，以确定重叠转录是否与基因间隔区的长度相关。同时，我们研究了兔脑炎微孢子虫中首次出现的转录重叠情况。利用蝗虫微孢子虫和兔脑炎微孢子虫之间基因顺序保守的基因组区域，我们确定了这两个物种的转录加工点，以确定该过程如何随进化时间而变化。我们表明，物种之间加工点的保守性很小，实际上这两个基因组中的过程在重要方面有所不同。总体而言，蝗虫微孢子虫的转录本通常在起始密码子上游开始，但在下游基因内部或下游基因之外很好地终止。相比之下，兔脑炎微孢子虫的转录本通常在上游基因内起始，但更频繁地在下游基因之前终止。这个过程在给定的基因组内似乎具有可预测的特征，但在物种之间相对灵活，这给这些专性细胞内寄生虫的基因表达研究带来了进一步的挑战。
