| dc.description |
The ASCE (additional strand, conserved E) superfamily of proteins consists of structurally similar ATPases associated with diverse cellular activities involving metabolism and transport of proteins and nucleic acids in all forms of life. A subset of these enzymes consists of multimeric ringed pumps responsible for DNA transport in processes including genome packaging in adenoviruses, herpesviruses, poxviruses and tailed bacteriophages. Although their mechanism of mechanochemical conversion is beginning to be understood, little is known about how these motors engage their nucleic acid substrates. Questions remain as to whether the motors contact a single DNA element, such as a phosphate or a base, or whether contacts are distributed over several parts of the DNA. Furthermore, the role of these contacts in the mechanochemical cycle is unknown. Here we use the genome packaging motor of the Bacillus subtilis bacteriophage Ï 29 (ref. 4) to address these questions. The full mechanochemical cycle of the motor, in which the ATPase is a pentameric-ring of gene product 16 (gp16), involves two phasesâ an ATP-loading dwell followed by a translocation burst of four 2.5-base-pair (bp) steps triggered by hydrolysis product release. By challenging the motor with a variety of modified DNA substrates, we show that during the dwell phase important contacts are made with adjacent phosphates every 10-bp on the 5â ²â 3â ² strand in the direction of packaging. As well as providing stable, long-lived contacts, these phosphate interactions also regulate the chemical cycle. In contrast, during the burst phase, we find that DNA translocation is driven against large forces by extensive contacts, some of which are not specific to the chemical moieties of DNA. Such promiscuous, nonspecific contacts may reflect common translocaseâ substrate interactions for both the nucleic acid and protein translocases of the ASCE superfamily. |
| Аффилиация |
Bustamante, Carlos; ;;;; Departments of Molecular and Cell Biology, Chemistry, and Howard Hughes Medical Institute, University of California, Berkeley, California 94720, USA |
