Cloning of CDK7 gene in Meretrix meretrix and its expression analysis on growth and development of different strains

【Objective】To master the spatiotemporal expression of cyclin dependent kinase 7（CDK7）gene（MmCDK7） in Meretrix meretrix and the expression law in the growth and development process of different strains ，so as to explore the growth advantages of new strains of red clam at the molecular level. It provided a theoretical basis for screening growth related genes of M. meretrix and revealing its growth and development mechanism.【Method】The cDNA sequence of MmCDK7 gene was cloned by RACE. Bioinformatics analysis was carried out by BLAST，ScanProsite，NetPhos 3.0 server and ExPASy. The expression of MmCDK7 gene in different tissues and different developmental stages of M. meretrix was detected by real-time fluorescence quantitative PCR. The differences of shell length，relative growth rate of shell length and MmCDK7 gene expression between red clam and yellow clam were compared under the same culture conditions.【Result】The total length of the cDNA sequence of MmCDK7 gene was 1296 bp，of which the 5 ' untranslated region（5'-UTR）was 83 bp，the 3 ' untranslated region（3'-UTR）was 196 bp，and the open reading frame（ORF）was 1017 bp，encoding 338 amino acid. The molecular weight of MmCDK7 protein was about 38.32 kD and the theoretical isoelectric point（pI）was 8.78. It included serine / threonine protein kinase catalytic domain（S_TKc），tyrosine kinase catalytic domain（TyrKc）and kinase domain NRTALRE related to cyclin binding. And S_ TKc domain contained protein kinase ATP binding site，serine / threonine protein kinase activation site and T-loop loop. The amino acid sequence of MmCDK7 was highly homologous with that of Mizuhopecten yessosensis，and its similarity was 75.00%. The phylogenetic tree constructed based on the amino acid sequence similarity of CDK7showed that M. meretrix and molluscs such as Octopus sinensis，Crassostrea gigas，Mytilus coruscus and M. yessosensis first gathered into one branch. MmCDK7 gene was expressed in gonad，siphon，mantle and hepatopancreas，and the relative expression in gonad was significantly higher than that in other tissues（P<0.05，the same below）. MmCDK7 gene was expressed in 8 developmental stages of two species，and the relative expression in multicellular stage was significantly higher than that in other stages. Under the same culture conditions，except September 18，the shell length of red clam was significantly longer than that of yellow clam，and the growth rate of red clam relative to yellow clam was 2.79%-32.37%. Except November 15 and November 30，the relative expression of MmCDK7 gene in red clam was significantly higher than that in yellow clam at other sampling time points.【Conclusion】MmCDK7 gene belongs to CDK family. The relative expression of CDK7 gene in gonad and multicellular stage with vigorous cell division is the highest，and the relative expression of CDK7 gene in red clam with fast growth rate is significantly higher than that in yellow clam in most cases. Therefore，it is speculated that MmCDK7 gene is involved in regulating the early growth and development of M. meretrix.