In bacteria, most genes are on the leading strand of replication, a phenomenon 
attributed to collisions between the DNA and RNA polymerases. In Escherichia 
coli, these collisions slow the movement of the replication fork through 
actively transcribed genes only if they are coded on the lagging strand. For 
genes on both strands, however, these collisions sever nascent transcripts and 
interrupt gene expression. Based on these observations, we propose a new theory 
to explain strand bias: genes whose expression is important for fitness are 
selected to the leading strand because this reduces the duration of these 
interruptions. Our theory predicts that multi-gene operons, which are subject to 
longer interruptions, should be more strongly selected to the leading strand 
than singleton transcripts. We show that this is true even after controlling for 
the tendency for essential genes, which are strongly biased to the leading 
strand, to occur in operons. Our theory also predicts that other factors that 
are associated with strand bias should have stronger effects for genes that are 
in operons. We find that expression level and phylogenetic ubiquity are 
correlated with strand bias for both essential and non-essential genes, but only 
for genes in operons.