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According to my medical physiology by Rhodes and Bell their description is as follows:

the essential light chain is necessary for myosin stability, and the other chain called the regulatory light chain is phosphorylated during muscle activity and serves to modulate muscle function*

However, according to molecular biology 4th edition:

it states that the light chains both exert their function via Calcium binding, they make no mention of phosphorylation.

So do light chains regulate myosin by calcium binding, phosphorylation or both ?

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The answer is both, the regulatory light chains , regulate myosin by phosphorylation which can be dependent or independent of $\ce{Ca^2+}$.

Medical Physiology:

The activity of the myosin regulatory light chain is in turn regulated through phosphorylation by $\ce{Ca^2+}$ -dependent and $\ce{Ca^2+}$ -independent kinases.

Source claiming calcium is essential for phosphorylation in tarantula skeletal muscle:

Contraction is modulated in many striated muscles by $\ce{Ca^2+}$-calmodulin dependent phosphorylation of the myosin regulatory light chain (RLC) by myosin light chain kinase.

Calcium independent phosphorylation of regulatory protein by MLCK4:

We find that MLCK4 is also expressed abundantly in cardiac muscle, and structural analyses indicate that it is a $\ce{Ca^2+}$/calmodulin (CaM)-independent kinase, in contrast to $\ce{Ca^2+}$/CaM-stimulated cMLCK.

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  • $\begingroup$ thank you for the answer! Does the calcium binding and/ or phosphorylation change the confirmation so as to modify the ATPase or the actin binding site ? $\endgroup$ Commented Jul 20, 2017 at 5:17
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    $\begingroup$ Read the section below figure 11.27 at ncbi.nlm.nih.gov/books/NBK9961 $\endgroup$
    – JM97
    Commented Jul 20, 2017 at 5:52

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