Expression of adenine nucleotide translocator isoform 2 (ANT2) is growth regulated. In the present study, we report the presence of a silencer region in the human ANT2 promoter and the purification of a two-component factor that recognizes a specific hexanucleotide element, GTCCTG, of the silencer. Transfection of deletion constructs shows that ANT2 silencer activity extends over a region of at least 310 nts. However, mutating the GTCCTG element completely relieves silencing activity in the context of the human ANT2 promoter. The data suggest that the GTCCTG element might be required for maintaining silencer activity of the extended silencer region. The ANT2 silencer region cloned in front of the herpes simplex virus thymidine kinase promoter confers nearly complete inhibition to the heterologous promoter. However, unlike the ANT2 promoter, mutating the GTCCTG element restores only partial activity to the herpes simplex virus thymidine kinase promoter. A protein complex consisting of two major polypeptides of 37 and 49 kDa was isolated from HeLa nuclear extracts by affinity chromatography using the GTCCTG element as the affinity resin. Cross-linking studies and Southwestern analysis indicate that p37 binds DNA. p49 appears to be loosely associated with the p37/DNA complex but is necessary for strong binding of p37. Our data implicating the GTCCTG element directly in silencing of the ANT2 promoter, together with data from the literature reporting the presence of this element within the silencer region of several additional promoters, suggest a general role of the GTCCTG element in transcriptional silencing.The adenine nucleotide translocator (ANT) 1 proteins exchange cytosolic ADP for mitochondrial ATP, thereby playing an essential role in maintaining cell metabolism and growth. Recent findings also implicate ANT in the initiation of events leading to apoptosis (1). Mammalian ANT is encoded in three genes, ANT1, ANT2, and ANT3 (2-5), that are expressed in a tissue-specific manner (6). ANT1 mRNA is expressed predominantly in heart and skeletal muscle (7, 8) whereas ANT2 mRNA is expressed in a broad range of tissues (6, 7, 9), but predominantly those tissues that undergo rapid proliferation (6, 10). However, the ANT2 isoform is unique in that it is also expressed in a growth-dependent manner (11) in a wide variety of cell lines. ANT2 expression is low in quiescent cells and is substantially increased by factors that induce entrance into G 1 and subsequent cell growth (11). ANT2 expression in serumactivated NIH3T3 cells is inhibited by actinomycin D 2 but not cycloheximide (12), indicating that expression is regulated at the level of transcription but does not require new protein synthesis.The physiological significance of expression of just the ANT2 isoform in a growth-dependent manner has not been delineated, although it is possible that slight kinetic differences in ADP/ATP exchange catalyzed by the isoforms (13) might provide the growth-activated cell with an energetic advantage. To understand the complex regula...

It is assumed that modern life forms arose from a molecular ancestor in which RNA molecules both stored genetic information and catalyzed biochemical reactions. In modern cells, these functions are carried out, respectively, by DNA and proteins, but diverse cellular RNAs are also involved in key cellular functions. In this paper, we review the cellular RNAs that are ubiquitous and/or that perform essential biological functions, and we discuss the evolutionary relationships of such RNAs with a prebiotic RNA world. This unexpected biological diversity of cellular RNAs and the crucial functions they perform in cellular metabolism demonstrate the complexity of an RNA-driven metabolism in an ancient RNA world and in modern life. Cellular RNAs are involved in translation (tRNA and rRNA) but also in ribosome maturation (snoRNA) and more generally in RNA processing (snRNA and snoRNA), replication (telomerase RNA), editing, protein translocation (SRP RNA), cellular transport (vRNA) and translation quality control (tmRNA). In addition, the function of many other cellular RNAs has not yet been determined. Future investigations of RNA function will allow us to better understand not only early evolutionary biological processes but also the central metabolism of modern cells.