siRNAs complementary to the P1 sequence. (A) Scheme of siRNAs complementary to the P1 sequence (P1-siRNAs) obtained from the cereal small RNA library (http://sundarlab.ucdavis.edu/smrnas/). A single copy of P1from B73 is represented. The P1-proximal enhancer is expanded to show the location of the P1-siRNAs identified. The siRNAs are shown as bars. Different colors represent different siRNA sizes, as indicated in the key. Asterisks show those siRNA assayed in the microarray experiment. (B) Validation of the data from the cereal small RNA library by microarray experiments using siRNAs from B73 leaves. Fifteen siRNAs were printed in the arrays as described in M&M. Only eight and two siRNAs corresponding to the proximal enhancer and 2nd intron of P1, respectively, showed hybridization values at least three times higher than the background. Mutator and Retrotransposon (gypsy-type) siRNAs were used as positive controls .