Serum lipoproteins as cholesterol donors to mycoplasma membranes.

Mycoplasma hominis andAcholeplasma laidlawii were grown in media in which a fraction of human serum lipoproteins provided the sole source of cholesterol. Increasing levels of very low density lipoproteins had an inhibitory effect on the growth of the organisms. Low and high density lipoproteins in all concentrations proved to be excellent sources of cholesterol. Both organisms were able to limit the amount of cholesterol taken up and to preferentially incorporate free cholesterol despite an excess of esterified cholesterol in the medium. When similar levels of free cholesterol were provided by low density or high density lipoproteins, the organisms incorporated from 20–45% more cholesterol from the former. This preference for cholesterol from low density lipoproteins partially supports the theory that the low density lipoproteins act as a donor while the high density lipoproteins are a scavenger of cholesterol.     

An unusual extended DNA loop attachment region is located in the human dystrophin gene

We have found and characterized an unusual extended area of DNA association with the nuclear matrix in the human dystrophin gene. This extended DNA loop anchorage region (LAR) has been mapped and characterized using a variety of biochemical and microscopy techniques. It spans approximately 200 kbp at chromosomal locations 950-1,150 Kb downstream to the beginning of the first exon of the dystrophin gene Dp427m and covers a part of the intron 43, exon 44, and most of intron 44. The extended LAR harbors the major recombination hot spot of the dystrophin gene and a replication origin. We propose a model where DNA topoisomerase II-mediated cleavage at the nuclear matrix may enhance recombination events within this extended LAR.     

TMEM8 – a non-globin gene entrapped in the globin web

For more than 30 years it was believed that globin gene domains included only genes encoding globin chains. Here we show that in chickens, the domain of α-globin genes also harbor the non-globin gene TMEM8. It was relocated to the vicinity of the α-globin cluster due to inversion of an ∼170-kb genomic fragment. Although in humans TMEM8 is preferentially expressed in resting T-lymphocytes, in chickens it acquired an erythroid-specific expression profile and is upregulated upon terminal differentiation of erythroblasts. This correlates with the presence of erythroid-specific regulatory elements in the body of chicken TMEM8, which interact with regulatory elements of the α-globin genes. Surprisingly, TMEM8 is not simply recruited to the α-globin gene domain active chromatin hub. An alternative chromatin hub is assembled, which includes some of the regulatory elements essential for the activation of globin gene expression. These regulatory elements should thus shuttle between two different chromatin hubs.     

Initiation of DNA replication in higher eukaryotes

In this review, of problems concerning initiation of DNA replication in higher eukaryotes is discussed, with special emphasis on the methods of replication origin mapping and biological tests for the activity of DNA replication origins in higher eukaryotes. Protein factors interacting with replication origins are considered in detail. The main events of replication initiation in higher eukaryotes are briefly analyzed. New data on the control of replication timing of large genomic regions are discussed.