Differential expression of cro, the lysogenic cycle repressor determinant of bacteriophage A2, in Lactobacillus casei and Escherichia coli

• The cro lysogenic cycle repressor of phage A2 gives rise to two colinear proteins.
• Cro* results from −1 frameshifting and is 12 residues shorter than the canonical Cro.
• No cro frameshifting was observed in A2-infected/induced L. casei cultures.
• This contradicts the suggested regulation of A2 life cycles by Cro–Cro* interaction.
• cro frameshifting is dependent on the genetic background and level of expression.

Expression of bacteriophage A2-encoded cro in Escherichia coli gives rise to two co-linear polypeptides, Cro and Cro*, which were proposed to form a regulatory tandem to modulate the frequency with which the phage would choose between the lytic and the lysogenic cycles. In this communication, it is reported that Cro is the canonical product of the gene cro while Cro* results from a −1 ribosome frameshift during translation and is twelve amino acids shorter than Cro. However, frameshifting was not observed during phage development in Lactobacillus casei. Furthermore, wild type phages and cro-frameshifting negative mutants present the same phenotype, thus corroborating that only the canonical form of Cro is needed to produce a viable phage progeny.