RETRACTION
Volume 19, no. 8, p. 5363–5372, 1999. The authors and the journal hereby retract this article. After publication, this article was found to have multiple images that were unacceptably manipulated (Fig. 1B, 4A to C, and 8A and B), a clear violation of ASM's ethical standards. Since the integrity of the data as presented was compromised, this publication is retracted in its entirety. We apologize to the readers of Molecular and Cellular Biology and regret any inconvenience this causes.