Post-transcriptional Gene Control Research Articles

Posttranscriptional Control of the Lysogenic Pathway in Bacteriophage Lambda

Publisher Summary This chapter discusses the recent developments in the study of Lambda (λ) phage gene expression showing that post-transcriptional control of phage genes by host functions provides highly sensitive regulatory circuits. The control of transcription initiation from the early promoters of bacteriophage λ is not sufficient to provide for subtle controls involved with the switch between lysogenic and lytic pathways. The ability of λ to choose between lytic and lysogenic development has evolved as a response to changes in the physiological state of the host cell. The frequency by which the phage enters the lytic or lysogenic pathway is determined in large measure by the nutritional state of the host cell. For example, starved cells lysogenize more efficiently than cells grown in a rich medium. In addition, the number of infecting phage particles per cell is also known to determine the rate of lysogenization: the higher the multiplicity of infection, the higher the rate of lysogenization. It is probable that the physiological state of the cell is signaled to the phage by host regulatory factors.

Post-transcriptional control of negative acute phase genes by transforming growth factor beta.

During the acute phase (AP) reaction the expression of a series of liver-specific genes coding for secretory proteins is either stimulated or suppressed by different cytokines released by activated monocytes. Transforming growth factor beta (TGF-beta) is a cytokine that, first identified for its ability to regulate cellular growth, has been gradually recognized to modulate several other functions. We have investigated the effect of TGF-beta on the expression of acute phase genes in liver cells. We found that TGF-beta selectively induces a specific decreases in the amount of mRNAs of genes negatively regulated during AP reaction, like albumin and apolipoprotein A-I (ApoA-I). The inhibitory effect of TGF-beta on the expression of negative AP genes is primarily post-transcriptional and it is very likely to be mediated via an enhancement of the turnover of both albumin and ApoA-I mRNAs.

There types of muscle-specific gene expression in fusion-blocked rat skeletal muscle cells: Translational control in EGTA-treated cells

When rat skeletal muscle cells were treated with EGTA, an inhibitor of cell fusion, a battery of muscle-specific mRNAs was synthesized but not translated despite the synthesis of many other proteins. Most of the muscle-specific mRNAs were associated with polysomes in fused myotubes, whereas they were found in postpolysomal fractions in EGTA-treated cells. Therefore, in addition to the well-documented transcriptional and posttranscriptional control of muscle-specific genes, translational control of this specific group of genes, presumably involving a Ca 2+-dependent process, is also observed in these fusion-blocked cells. These findings and results obtained with other fusion inhibitors demonstrate that three types of muscle-specific gene expression take place in the fusion-blocked cells depending on the inhibitors used: one, neither muscle-specific mRNAs nor proteins are synthesized; two, the mRNAs are synthesized but not translated; and three, both the mRNAs and the proteins are synthesized.

Immunoglobulin RNA Rearrangements in B Lymphocyte Differentiation

Publisher Summary DNA rearrangements generate transcriptionally active light and heavy chain genes. These DNA rearrangements are unique among the higher eukaryotic genes studied to date. In conjunction with multiple germline, variable region genes and yet unresolved mechanisms that generate somatic mutations in variable regions, these DNA joining events contribute significantly to the immense diversity of antibodies. This chapter reviews recent findings that reveal the importance of RNA splicing and processing rearrangements in the expression and regulation of immunoglobulin heavy chain genes. RNA rearrangements greatly increase the versatility of immunoglobulin heavy chain genes by generating alternate forms of mRNA, with different functions, from one transcription unit. Both secreted and membrane mRNA species for all heavy chain classes are generated by alternative RNA splicing pathways. The immunoglobulin molecule can exist in two very different environments: (1) in the lymphocyte cell membrane as a surface antigen receptor and (2) in the circulation as a secreted antibody. Most secreted immunoglobulin molecules consist of two identical light chains and two identical heavy chains. Each chain is composed of a series of homologous domains, the NH2-terminal one being the variable (V) region and the remainder forming the constant (C) region. Complex transcription units and posttranscriptional control of chromosomal genes are not unique to immunoglobulin heavy chain genes. The findings collected in the chapter clearly establish the importance of posttranscriptional RNA processing mechanisms in regulation of genes in eukaryotic cells.

Transcriptional and post-transcriptional control of ribosomal protein and ribonucleic acid polymerase genes.

A partial restriction of ribonucleic acid (RNA) polymerase activity has been used to dissociate the coordinate synthesis of ribosomal proteins and subunits of RNA polymerase and to identify transcriptional and post-transcriptional control signals which regulate the expression of these component genes. Within the beta operon [which has the genetic organization: promoter (p beta), rplJ (L10), r;lL (L7/L12), attenuator, rpoB (beta), rpoC (beta'), terminator], the restriction caused a disproportionate increase between proximal and distal gene transcriptions; the transcriptional intensities of the proximal ribosomal protein genes and the distal RNA polymerase genes were elevated about two- and fourfold, respectively. Transcription within the operon containing four ribosomal protein genes and the RNA polymerase alpha gene was also enhanced, whereas transcription within operons containing only ribosomal protein genes was virtually unaffected by the restriction. It was thus concluded that the mechanisms controlling transcription initiation or attenuation or both in operons containing RNA polymerase subunit genes are coupled to the global rate of RNA synthesis. By introducing the composite ColE1 plasmid pJC701 carrying the proximal portion of the L10 operon, including the beta subunit gene, it was possible to achieve a 10- and a 30-fold range in the transcriptional intensities of the genes specifying L10 and L7/L12 and beta, respectively. Under these conditions, the relative synthesis rates of L7/L12 and beta protein varied by less than 2-fold and by about 15-fold, respectively. These observations corroborate the existence of a post-transcriptional mechanism which severely restricts translation of excess L7/L12 and L10 ribosomal protein messenger RNA; this mechanism is probably important in maintaining the balanced synthesis of ribosome components under conditions in which their messenger RNA levels are dissociated. Furthermore, the observed reduction in the translation efficiency of beta subunit messenger RNA may be related to an inhibitory effect caused by accumulation of RNA polymerase assembly intermediates.

Transcriptional and post-transcriptional control of RNA polymerase and ribosomal protein genes cloned on composite ColE1 plasmids in the bacterium Escherichia coli.

Composite ColEl plasmids carrying some or all of the bacterial genes from near 88 min on the Escherichia coli chromosome which specify the four ribosomal proteins Lll, Ll, LlO, and L7/L12, and the /3 and p’ subunits of RNA polymerase have been constructed (Collins, J., Fiil, N., Jorgensen, P., and Friesen, J. (1976) in Alfred Benzon Symposium IX, Control of Macromolecular Synthesis (Kjeldgaard, N. O., and Maaloe, O., eds) pp. 357369, Munksgaard Press, Copenhagen). These plasmids exhibit relaxed replication and result in a 7to It-fold amplification in the copy number of the respective ribosomal protein and RNA polymerase subunit genes in the plasmid-containing strains. These strains were used to study the effect of amplification on (i) the transcriptional expression of these genes and (ii) the subsequent translation of the mRNAs to yield the respective protein products. Our results demonstrate that amplification of DNA sequences specifying ribosomal proteins LlO and L7/L12 and RNA polymerase fi and p’ subunits, along with their promotor, increases transcription of these genes by about 6-fold and results in elevated levels of the respective mRNA sequences. In these plasmid strains the synthesis rate of L7/L12 ribosomal protein was estimated to be only about 30% greater than the synthesis rate in non-plasmid control strain. It was further apparent that about two-thirds of the total L7/L12 protein was specified by the L7/L12 plasmid gene and one-third by the L7/L12 chromosomal gene. The synthesis rates of /I and /Y RNA polymerase subunits were elevated only about a-fold when these genes were amplified. These observations indicate the existence of a post-transcriptional control mechanism which limits the overproduction of both ribosomal protein L7/L12 and the /I and /l’ subunits of RNA polymerase by restricting translation of the elevated levels of mRNA produced by amplification of the respective genes.