A recent study of transgene expression in plants revisits themes that recur in the literature every so often. Namely, that “alternative polyadenylation” and low-level transcription well beyond a plant polyadenylation signal are common occurrences in plants. These phenomena are not limited to just transgenes, but are seen with most (if not all) genes.
Why the scare quotes? Because, while the events documented in this study are formally occurrences of alternative polyadenylation, they reflect the inherent 3′ end microheterogeneity that is seen in almost all plant genes, and probably involves subtly different handling of a single polyadenylation signal. This sort of poly(A) site heterogeneity should be distinguished from the occurrence of clearly distinct polyadenylation signals, separated by hundreds or thousands of nucleotides along a transcript.
In any case, this short report reinforces the notion that transcriptional readthrough from transgenes can impact the expression of “host” genes via posttranscriptional gene silencing.
Plant genetic engineering can create transgenic crops with improved characteristics by introducing trait genes through transformation. Appropriate regulatory elements such as promoters and terminators have to be present in certain configurations for the transgenes to be properly expressed. Five terminators native to soybean genes-encoding a MYB family transcription factor (MYB2), a Kunitz trypsin inhibitor (KTI1), a plasma membrane intrinsic protein (PIP1), a translation elongation factor (EF1A2) and a metallothionein protein (MTH1) were cloned and tested for their ability to enable transgene expression, mRNA polyadenylation and transcription termination. The terminators are as good as a control terminator of the potato proteinase inhibitor II gene (PINII) in conferring proper transgene expression, leading to mRNAs with various polyadenylation sites and terminating mRNA transcripts. RNA transcription read-through was detected in all transgenic plants and was quantified by qRT-PCR to be <1% at positions ∼1 kb downstream of the 5′ ends of different terminators. The detection of read-through RNA transcripts of the corresponding endogenous genes up to approximately 1 kb beyond the polyadenylation sites suggests that limited RNA transcription read-through is a normal phenomenon of gene expression. The study also provided more choices of terminators for plant genetic engineering when constructing DNA constructs containing multiple gene expression cassettes.
Xing A, Moon BP, Mills KM, Falco SC, Li Z. Revealing frequent alternative polyadenylation and widespread low-level transcription read-through of novel plant transcription terminators. Plant Biotechnol J. 2010 Mar 16. [Epub ahead of print]
A few other recent studies that deal with transcription termination in transgenic plants:
Nicholson SJ, Srivastava V, Transgene constructs lacking transcription termination signal induce efficient silencing of endogenous targets in Arabidopsis. Mol Genet Genomics. 2009 Sep;282(3):319-28.
Yang L, Wakasa Y, Kawakatsu T, Takaiwa F. The 3′-untranslated region of rice glutelin GluB-1 affects accumulation of heterologous protein in transgenic rice. Biotechnol Lett. 2009 Oct;31(10):1625-31.
Misztal LH, Mostowska A, Skibinska M, Bajsa J, Musial WG, Jarmolowski A. Expression of modified Cry1Ac gene of Bacillus thuringiensis in transgenic tobacco plants. Mol Biotechnol. 2004 Jan;26(1):17-26.