2. Materials and methods
2.1. Biological sample
To maximize the diversity of transcriptional units sampled, a total
of 10 healthy individuals of A. japonicus (body weight: 9.3–10.1 g)
were collected from Guanglu Island, Dalian, China. The body walls,
intestines and respiratory trees were removed and frozen immediately
with liquid nitrogen and then stored at−80 °C for RNA extraction
and cDNA library construction.
2.2. Construction and characterization of cDNA libraries
For each tissue, total RNA was extracted from a pool of equal
proportion of materials from all ten individuals using Takara RNAiso
Reagent (Takara). mRNA was isolated using oligotex-dT30 and mRNA
purification kit (Takara). cDNA synthesis was conducted using 100 ng
of mRNA with Creator SMART cDNA library construction kit
(Clontech). The double-stranded cDNA was ligated into pDNR-LIB
vector in a sense direction (Sfi IA/Sfi IB) and was electro-transformed
into the Escherichia coli strain DH10B. Insert size was determined by
PCR with the vector primer pair T3/T7. The following cycling
conditions were used: 94 °C for 1 min 20 s; 35 cycles at 94 °C for
1 min, 50 °C for 1 min , 72 °C for 1 min 30 s; followed by a final
incubation step of 5 min at 72 °C. The PCR products were separated on
1.0% agarose gels, stained with ethidium bromide and visualized
under UV light.
2.3. Plasmid preparation and DNA sequencing
Individual colonies were randomly picked from the cDNA libraries
and were grown in 96-well plates at 37 °C for 18 h. Bacterial plasmids
were extracted by conventional method and stored at−20 °C prior to
sequencing. Single-pass nucleotide sequence at the 5′-end of each
cDNA was obtained using a T7 universal primer and a 3730XL DNA
analyzer (Applied Biosystems).
2.4. Sequence analysis, contig assembly, and functional categorization
Quality control of sequenced ESTs was performed using the Phred
program with a cut-off of 20 for trimming low quality regions, and
Cross-Match for vector trimming (Ewing et al., 1998; Ewing and
Green, 1998). High-quality ESTs (N100 bp) were then assembled and
clustered into contiguous sequences (contigs) using the Phrap
program set to the default parameters (Gordon et al., 2001). Unigenes
including contigs and singletons were subjected to the NCBI
nonredundant (nr) protein databases for annotation by performing
BLASTX with a cut-off E value of the best hit of ≤10−5. Sequences
without a reliable match (N10−5) were subsequently compared to the
NCBI nt database by performing BLASTN for complementary annotation.
Gene Ontology (GO) information was obtained according to its
UniGene accession number or symbol in the GO Consortium (available
at http://www.geneontology.org/) (Ashburner et al., 2000).