Abstract
Abstract
Integrative and conjugative elements (ICEs) are widespread in bacteria and typically confer important phenotypes to their hosts. Following excision from the host chromosome, an ICE can transfer to a new cell through the element-encoded conjugation machinery (a type IV secretion system) to create a transconjugant. Tn
916
, a well-studied ICE that confers tetracycline resistance, is found in several Gram-positive pathogens and has been widely studied in
Bacillus subtilis
. Unlike most ICEs, Tn
916
causes host-cell death following its activation (excision and gene expression). We found that
orf7
and
orf8
(renamed
sig7
and
sig8
), two regulatory genes in Tn
916
, encode a two-subunit (split) sigma factor of RNA polymerase, Sig7-8, that activates transcription from P
orf7
(now P
sig7
) and is required for excision of the element. Importantly, production of Sig7-8, either from P
sig7
or another strong promoter, caused cell death. Using a Cre recombinase reporter to detect gene expression, we detected little or no transcription from P
sig7
in transconjugants. This lack of transcription from P
sig7
in transconjugants was essential for fitness of both the element and its host: forcing transcription from P
sig7
in transconjugants caused either loss of Tn
916
or death of nascent transconjugants. Our findings indicate that, unlike other well-studied ICEs (e.g., ICE
Bs1
) that use repressors to control gene expression and are expressed in nascent transconjugants, most Tn
916
genes remain off following transfer to a new host. This regulation prevents Tn
916
from killing transconjugants and allows successful horizontal transfer and maintains the potential for future activation in the new host.