What is a poly(T) primer anchor sequence? I am reading Tang et al. (2009).

In the method description it is stated that:


  A single cell is manually picked under a microscope and lysed. Then mRNAs are reverse-transcribed into cDNAs using a poly(T) primer with anchor sequence (UP1) and unused primers are digested.


What is an anchor sequence in this case?


Tang, F. et al. mRNA-Seq whole-transcriptome analysis of a single cell. Nat. Methods 6, 377–382 (2009).
As shown in figure 1 of that paper, the anchor sequence is additional sequence that is added to the end of the polyT primer that provides a known and non-target binding site for additional PCR reactions. I gave a presentation about a month ago about the process that Oxford Nanopore Technology use in their strand-switching cDNA kit, which may be useful for understanding the process used by Tang et al.. Here are my strand-switch/PCR images from that presentation:


PolyT primer addition, with mismatched anchor sequence





Strand switch primer addition, with overhanging anchor sequence





PCR amplification, using primers that are complementary to the anchor sequences




In the case of the Tang et al. paper, they use a polyA-tailing method for strand-switching, but the process is essentially the same.