MicroRNAs (miRNAs) are a newly identified class of endogenous, non-coding, short ( approximately 21nt) RNAs that play important roles in regulating gene expression at post-transcriptional level by targeting mRNA cleavage or translational inhibition in plants and animals. Though there are lots of differences between plant miRNAs and animal miRNAs, most of these tiny RNAs are highly conserved in each kingdom. Here, we show the conserved miRNAs in lettuce (Lactuca) identified using EST (expressed sequence tag) analysis. Namely, all previously known miRNAs in other plant species were blasted against lettuce EST sequences to select novel miRNAs in lettuce by a series of filtering criteria. By this strategy, we found a total of 21 conserved miRNAs belonging to 12 miRNA families. After analyzing the conservation and evolution of lettuce miRNAs and their counterparts in other plant species, we revealed that though miRNAs are highly conserved, some specific sites are more likely to mutate. To confirm the expression of identified miRNAs in lettuce, an RT-PCR approach was employed. Moreover, all identified lettuce miRNAs were used to search their potential target genes by miRU web-server from TIGR database available at http://www.tigr.org and a total of 63 potential targets for 10 identified miRNA families in lettuce were found. Similar to previous works, some miRNA targets are transcription factors involved in lettuce growth and development, metabolism, and stress responses.
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