We determined the fidelity of avian myeloblastosis virus and Moloney murine leukemia virus reverse transcriptases (RTs) during DNA synthesis in vitro using the M13mp2 lacZa gene as a mutational target Retroviral reverse transcriptases (RTs) (EC 2.7.7.7) are DNA polymerases that transcribe retroviral genomic RNA into double-stranded DNA. RT was first identified in virions of murine leukemia virus (MuLV) by Baltimore (1) and Rous sarcoma virus by Temin and Mizutani (54). These enzymes perform multiple enzymatic functions, including the copying of RNA into DNA, the hydrolysis of the RNA from an RNA-DNA hybrid (RNase H), and the copying of singlestranded DNA into double-stranded DNA; thus, RT plays a central role in the life cycle of retroviruses (55,56).The physical structure of purified RT from avian and murine retroviruses has been studied extensively (for a review, see reference 58). Avian retrovirus RT is a heterodimeric molecule containing two related subunits, one with a molecular size of -63 kilodaltons (the a subunit) and the other -95 kilodaltons (the ( subunit) (18). These two proteins, along with a pp32Pol phosphoprotein endonuclease, are derived by differential processing of a pol polyprotein precursor (45). The endonuclease specifically nicks DNA at the junction of adjacent retroviral long terminal repeat sequences (14). The a subunit of avian RT appears to carry both polymerase and RNase H activities.The murine retroviral RT differs from the avian form in that it is a single 84-kilodalton polypeptide (57). Finestructure mutational analysis of the murine enzyme indicated that both the polymerase and the RNase H activities are contained in this polypeptide, with the polymerase activity mapping to the N terminus and the RNase H activity at the C terminus (52). Thus, the murine RT appears to be analogous to the a subunit of the avian RT.

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Salmonella enterica serovar Typhimurium possesses a stimulon of genes that are differentially regulated in response to conditions of low fluid shear force that increase bacterial virulence and alter other phenotypes. In this study, we show that a previously uncharacterized member of this stimulon, ydcI or STM1625, encodes a highly conserved DNA binding protein with related homologs present in a range of Gram-negative bacterial genera. Gene expression analysis shows that ydcI is expressed in different bacterial genera and is involved in its autoregulation in S. Typhimurium. We demonstrate that purified YdcI protein specifically binds a DNA probe consisting of its own promoter sequence. We constructed an S. Typhimurium ⌬ydcI mutant strain and show that this strain is more sensitive to both organic and inorganic acid stress than is an isogenic WT strain, and this defect is complemented in trans. Moreover, our data indicate that ydcI is part of the rpoS regulon related to stress resistance. The S. Typhimurium ⌬ydcI mutant was able to invade cultured cells to the same degree as the WT strain, but a strain in which ydcI expression is induced invaded cells at a level 2.8 times higher than that of the WT. In addition, induction of ydcI expression in S. Typhimurium resulted in the formation of a biofilm in stationary-phase cultures. These data indicate the ydcI gene encodes a conserved DNA binding protein involved with aspects of prokaryotic biology related to stress resistance and possibly virulence.Bacterial growth environments characterized by low fluid shear force have been shown to induce a multitude of phenotypic responses, including altered acid, oxidative, thermal, and osmotic stress resistance (7,33,36,41,(52)(53)(54)(55), increased biofilm formation (6, 33, 52), altered protein secretion (14, 15), altered cell surface lipid and polysaccharide profiles (6, 7, 55), and increased survival in cellular and animal hosts (39,(52)(53)(54). Notably, the virulence of Salmonella enterica serovar Typhimurium is increased by low fluid shear growth conditions as measured using murine infection assays and tissue culture models (39,(52)(53)(54). Low fluid shear force (defined here as approximately Ͻ0.01 to 0.2 dynes/cm 2 ) is characterized by a lowturbulence, low-agitation environment, as opposed to high fluid shear (defined here as approximately from 5 to Ͼ50 dynes/cm 2 ) where liquid moves with higher velocity over the cellular surface (3-5, 21, 23, 25, 36). Low fluid shear growth environments include spaceflight, ground-based suspension culture models such as the rotating-wall vessel (RWV) bioreactor, and the spaces between cellular microvilli, the last of which is encountered by numerous pathogens during the natural course of infection (21,23,25,30,40,41). Previous work has shown that bacteria grown in low fluid shear environments induce a molecular response which includes genome-wide changes in gene expression (the low fluid shear stimulon) (6,7,39,48,52,53,55). Since growth under low fluid shear conditions is able to ind...

Supplementary data are available at Bioinformatics online.