Role for 10Sa RNA in the growth of lambda-P22 hybrid phage

Certain lambda-P22 hybrids, providing that they express the P22 C1 protein, fail to grow in Escherichia coli with the sipB391 mutation. We show that sipB391, previously located to the 57-min region of the E. coli chromosome, is a large deletion that extends into the 3' end of ssrA, a gene encoding the small stable 10Sa RNA. This deletion, apparently created by the excision of a cryptic prophage, CP4-57 (identified by Kirby et al. J. E. Kirby, J. E. Trempy, and S. Gottesman, J. Bacteriol. 176:2068-2081), leaves most of ssrA intact but removes the sequence encoding the 3' end of the precursor form of 10Sa RNA. The lack of functional 10Sa RNA, resulting from either the excision of CP4-57 or insertional inactivation of ssrA, appears to be responsible for the inhibition of lambda-P22 growth in E. coli with the sipB391 mutation. We propose that 10Sa RNA acts either directly or indirectly to facilitate removal of C1 protein from its DNA target site.