Sudhakar B, Gopinathan K P
Microbiology and Cell Biology Department, Indian Institute of Science, 560 012, Bangalore, India.
Gene. 2000 Oct 17;257(1):77-85. doi: 10.1016/s0378-1119(00)00391-7.
The silk glands of mulberry silkworm Bombyx mori are endoreplicating tissues in which the genomic DNA undergoes multiple rounds of replication without mitosis and nuclear division. In the absence of normal mitotic division, the cell cycle essentially alternates between the G1 and S phases. Cyclin E is crucial for the G1/S transition in both mitotic and endoreplicating cycles. We have cloned and characterized cyclin E (cyclin box) from B. mori, which is nearly identical to the Drosophila cyclin E box except for an insertion of 21 amino acids. Two distinct cyclin E transcripts (1.7 and 2.1 kb) were detected in the silk-gland cells of B. mori and in the B. mori-derived embryonic cell line, BmN. Using anti Cyclin E antibodies two protein bands of 52 and 44kDa were detected in silk glands and BmN cells at comparable levels. Both BmN- and the silk-gland cells showed the presence of the interacting kinase Cdk2. Transcripts of the mitotic cyclin, cyclin B, were barely detectable in the endoreplicating silk-gland cells and amounted to only 4-7% of that seen in the mitotically dividing BmN cells. The near absence of cyclin B transcripts and the abundant expression of cyclin E in the silk glands correlate well with the alternation of only G1 and S phases without the intervening mitosis in these cells.
家蚕的丝腺是进行核内复制的组织，其中基因组DNA经历多轮复制，而无有丝分裂和核分裂。在缺乏正常有丝分裂的情况下，细胞周期基本上在G1期和S期之间交替。细胞周期蛋白E对于有丝分裂和核内复制周期中的G1/S转换至关重要。我们已经从家蚕中克隆并鉴定了细胞周期蛋白E（细胞周期蛋白框），它与果蝇细胞周期蛋白E框几乎相同，只是插入了21个氨基酸。在家蚕的丝腺细胞和家蚕来源的胚胎细胞系BmN中检测到两种不同的细胞周期蛋白E转录本（1.7和2.1 kb）。使用抗细胞周期蛋白E抗体，在丝腺和BmN细胞中检测到52 kDa和44 kDa的两条蛋白带，水平相当。BmN细胞和丝腺细胞均显示存在相互作用的激酶Cdk2。有丝分裂细胞周期蛋白细胞周期蛋白B的转录本在进行核内复制的丝腺细胞中几乎检测不到，仅为有丝分裂的BmN细胞中所见转录本的4-7%。丝腺中细胞周期蛋白B转录本几乎不存在而细胞周期蛋白E大量表达，与这些细胞中仅G1期和S期交替而无中间有丝分裂的情况密切相关。
