植物基因在转录水平上的调控及其生物学意义

植物发生变异或分化的本质是基因表达模式发生变化的结果, 而基因表达多是在转录水平进行调控的。文章通过分析总结前人在这一领域内的大量研究成果, 系统地从遗传学（genetics）和表观遗传学（epigenetics）两个角度对植物基因在转录水平上的调控方式及其生物学意义进行了总结性阐述, 分析了这一研究领域目前所面临的挑战, 展望了该领域今后的发展及应用前景。     

Transcriptional regulation of HLA class II and invariant chain genes

Class II (Ia) antigens are coded for by a family of genes located in the human MHC (HLA). These genes are regulated in a complex manner, being constitutively expressed, inducibly expressed, or not expressed, depending on the cell type examined. 6.1.6 is a variant of a normal B lymphoblastoid line that has lost expression of all class II molecules and has previously been shown to have a defect in the regulation of class II genes. In this report, we have examined those genes by Southern and Northern blotting and have found that 6.1.6 is severely deficient in mRNA for all class II genes examined, although the genes are structurally intact. P30, a partial revertant of 6.1.6, re-expresses mRNA for a subset of class II genes. mRNA for the class II-associated invariant chain is substantially reduced but not absent in 6.1.6.     

Transcriptional regulation of nitrogen fixation genes by DNA supercoiling

DNA gyrase (ATP dependent topoisomerase type II, EC 5.99.1.3) was found to be essential for the expression of the Klebsiella pneumoniae nitrogen fixation gene cluster carried by plasmid pRD1 in Escherichia coli. In the absence of DNA gyrase activity, nitrogen fixation activity could be restored by providing a constitutively expressed nifA function in trans. Our results suggest that nif gene regulation by oxygen may be mediated through the alteration of the superhelical status of the promoter of the nifLA regulatory operon, in addition to the action of the nifL gene product.Communicated by J. Schell     

Transcriptional regulation of lux genes transferred into Vibrio harveyi

Past work has shown that transformed Escherichia coli is not a suitable vehicle for studying the expression and regulation of the cloned luminescence (lux) genes of Vibrio harveyi. Therefore, we have used a conjugative system to transfer lux genes cloned into E. coli back into V. harveyi, where they can be studied in the parental organism. To do this, lux DNA was inserted into a broad-spectrum vector, pKT230, cloned in E. coli, and then mobilized into V. harveyi by mating aided by the conjugative plasmid pRK2013, also contained in E. coli. Transfer of the wild-type luxD gene into the V. harveyi M17 mutant by this means resulted in complementation of the luxD mutation and full restoration of luminescence in the mutant; expression of transferase activity was induced if DNA upstream of luxC preceded the luxD gene on the plasmid, indicating the presence of a strong inducible promoter. To extend the usefulness of the transfer system, the gene for chloramphenicol acetyltransferase was inserted into the pKT230 vector as a reporter. The promoter upstream of luxC was verified to be cell density regulated and, in addition, glucose repressible. It is suggested that this promoter may be the primary autoregulated promoter of the V. harveyi luminescence system. Strong termination signals on both DNA strands were recognized and are located downstream from luxE at a point complementary to the longest mRNA from the lux operon. Structural lux genes transferred back into V. harveyi under control of the luxC promoter are expressed at very high levels in V. harveyi as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis: the gene transfer system is thus useful for expression of proteins as well as for studying the regulation of lux genes in their native environment.