Mutations in an Abf1p binding site in the promoter of yeast RPO26 shift the transcription start sites and reduce the level of RPO26 mRNA

Two Routes to Genetic Suppression of RNA Trimethylguanosine Cap Deficiency via C-Terminal Truncation of U1 snRNP Subunit Snp1 or Overexpression of RNA Polymerase Subunit Rpo26

The trimethylguanosine (TMG) caps of small nuclear (sn) RNAs are synthesized by the enzyme Tgs1 via sequential methyl additions to the N2 atom of the m⁷G cap. Whereas TMG caps are inessential for Saccharomyces cerevisiae vegetative growth at 25° to 37°, tgs1∆ cells that lack TMG caps fail to thrive at 18°. The cold-sensitive defect correlates with ectopic stoichiometric association of nuclear cap-binding complex (CBC) with the residual m⁷G cap of the U1 snRNA and is suppressed fully by Cbc2 mutations that weaken cap binding. Here, we show that normal growth of tgs1∆ cells at 18° is also restored by a C-terminal deletion of 77 amino acids from the Snp1 subunit of yeast U1 snRNP. These results underscore the U1 snRNP as a focal point for TMG cap function in vivo. Casting a broader net, we conducted a dosage suppressor screen for genes that allowed survival of tgs1∆ cells at 18°. We thereby recovered RPO26 (encoding a shared subunit of all three nuclear RNA polymerases) and RPO31 (encoding the largest subunit of RNA polymerase III) as moderate and weak suppressors of tgs1∆ cold sensitivity, respectively. A structure-guided mutagenesis of Rpo26, using rpo26∆ complementation and tgs1∆ suppression as activity readouts, defined Rpo26-(78-155) as a minimized functional domain. Alanine scanning identified Glu89, Glu124, Arg135, and Arg136 as essential for rpo26∆ complementation. The E124A and R135A alleles retained tgs1∆ suppressor activity, thereby establishing a separation-of-function. These results illuminate the structure activity profile of an essential RNA polymerase component.

A cis-regulatory mutation in troponin-I of Drosophila reveals the importance of proper stoichiometry of structural proteins during muscle assembly

Rapid and high wing-beat frequencies achieved during insect flight are powered by the indirect flight muscles, the largest group of muscles present in the thorax. Any anomaly during the assembly and/or structural impairment of the indirect flight muscles gives rise to a flightless phenotype. Multiple mutagenesis screens in Drosophila melanogaster for defective flight behavior have led to the isolation and characterization of mutations that have been instrumental in the identification of many proteins and residues that are important for muscle assembly, function, and disease. In this article, we present a molecular-genetic characterization of a flightless mutation, flightless-H (fliH), originally designated as heldup-a (hdp-a). We show that fliH is a cis-regulatory mutation of the wings up A (wupA) gene, which codes for the troponin-I protein, one of the troponin complex proteins, involved in regulation of muscle contraction. The mutation leads to reduced levels of troponin-I transcript and protein. In addition to this, there is also coordinated reduction in transcript and protein levels of other structural protein isoforms that are part of the troponin complex. The altered transcript and protein stoichiometry ultimately culminates in unregulated acto-myosin interactions and a hypercontraction muscle phenotype. Our results shed new insights into the importance of maintaining the stoichiometry of structural proteins during muscle assembly for proper function with implications for the identification of mutations and disease phenotypes in other species, including humans.

Induced expression, localization, and chromosome mapping of a gene for the TBP-interacting protein 120A

TBP-interacting protein 120A (TIP120A) is a novel eukaryotic transcriptional regulator and has been suggested to be involved in the general regulation of transcription because of its ability to potentiate transcription of all classes of genes and to interact with common transcriptional machineries. In the present study, we investigated the expression of the tip120a gene. TIP120A transcripts were expressed abundantly in the heart and liver, moderately in the brain and skeletal muscle, and only slightly in the spleen and lung. This ubiquitous expression pattern was similar to that of TBP. Gene expression of TIP120A in the rat liver was not stimulated by hepatocarcinogenesis or liver regeneration. TIP120A was thus suggested not to be a growth-related protein. On the other hand, in P19 mouse embryonal carcinoma cells, TIP120A expression was elevated upon retinoic acid treatment, which induces differentiation. Notably, the foci-like nuclear localization pattern of TIP120A was transformed into a speckle-like pattern. The level of TIP120A was also elevated in such stem-like cells as F9 and HL60 after each differentiation procedure, retinoic acid and DMSO, respectively. In HEp-2 cells, TIP120A was observed as a limited number of nuclear foci, and the localization coincided with that of the PML oncogenic domain. FISH detection revealed that the human tip120a gene was located at 12q14, the position to which a myopathic type scapuloperoneal syndrome locus also mapped. Our study suggests that, contrary to an early assumption, TIP120A is involved in tissue-specific and/or differentiation-related gene expression.